diff --git a/src/pyconstraint.c b/src/pyconstraint.c
index 7845eab..8a8ce0a 100644
--- a/src/pyconstraint.c
+++ b/src/pyconstraint.c
@@ -95,25 +95,25 @@ PyObject *_ped_Constraint_richcompare(_ped_Constraint *a, PyObject *b, int op) {
 
 PyObject *_ped_Constraint_str(_ped_Constraint *self) {
     char *ret = NULL;
-    char *start_align = NULL, *end_align = NULL;
-    char *start_range = NULL, *end_range = NULL;
+    const char *start_align = NULL, *end_align = NULL;
+    const char *start_range = NULL, *end_range = NULL;
 
-    start_align = (char *) PyUnicode_AsUTF8(_ped_Alignment_Type_obj.tp_repr(self->start_align));
+    start_align = PyUnicode_AsUTF8(_ped_Alignment_Type_obj.tp_repr(self->start_align));
     if (start_align == NULL) {
         return NULL;
     }
 
-    end_align = (char *) PyUnicode_AsUTF8(_ped_Alignment_Type_obj.tp_repr(self->end_align));
+    end_align = PyUnicode_AsUTF8(_ped_Alignment_Type_obj.tp_repr(self->end_align));
     if (end_align == NULL) {
         return NULL;
     }
 
-    start_range = (char *) PyUnicode_AsUTF8(_ped_Geometry_Type_obj.tp_repr(self->start_range));
+    start_range = PyUnicode_AsUTF8(_ped_Geometry_Type_obj.tp_repr(self->start_range));
     if (start_range == NULL) {
         return NULL;
     }
 
-    end_range = (char *) PyUnicode_AsUTF8(_ped_Geometry_Type_obj.tp_repr(self->end_range));
+    end_range = PyUnicode_AsUTF8(_ped_Geometry_Type_obj.tp_repr(self->end_range));
     if (end_range == NULL) {
         return NULL;
     }
diff --git a/src/pydevice.c b/src/pydevice.c
index 20f492e..6c09d08 100644
--- a/src/pydevice.c
+++ b/src/pydevice.c
@@ -186,14 +186,14 @@ PyObject *_ped_Device_richcompare(_ped_Device *a, PyObject *b, int op) {
 
 PyObject *_ped_Device_str(_ped_Device *self) {
     char *ret = NULL;
-    char *hw_geom = NULL, *bios_geom = NULL;
+    const char *hw_geom = NULL, *bios_geom = NULL;
 
-    hw_geom = (char *) PyUnicode_AsUTF8(_ped_CHSGeometry_Type_obj.tp_repr(self->hw_geom));
+    hw_geom = PyUnicode_AsUTF8(_ped_CHSGeometry_Type_obj.tp_repr(self->hw_geom));
     if (hw_geom == NULL) {
         return NULL;
     }
 
-    bios_geom = (char *) PyUnicode_AsUTF8(_ped_CHSGeometry_Type_obj.tp_repr(self->bios_geom));
+    bios_geom = PyUnicode_AsUTF8(_ped_CHSGeometry_Type_obj.tp_repr(self->bios_geom));
     if (bios_geom == NULL) {
         return NULL;
     }
diff --git a/src/pydisk.c b/src/pydisk.c
index 8856f8e..a67f43f 100644
--- a/src/pydisk.c
+++ b/src/pydisk.c
@@ -98,7 +98,7 @@ PyObject *_ped_Partition_richcompare(_ped_Partition *a, PyObject *b, int op) {
 
 PyObject *_ped_Partition_str(_ped_Partition *self) {
     char *ret = NULL;
-    char *disk = NULL, *fs_type = NULL, *geom = NULL;
+    const char *disk = NULL, *fs_type = NULL, *geom = NULL;
 
     disk = (char *) PyUnicode_AsUTF8(_ped_Disk_Type_obj.tp_repr(self->disk));
     if (disk == NULL) {
@@ -345,7 +345,7 @@ PyObject *_ped_Disk_richcompare(_ped_Disk *a, PyObject *b, int op) {
 
 PyObject *_ped_Disk_str(_ped_Disk *self) {
     char *ret = NULL;
-    char *dev = NULL, *type = NULL;
+    const char *dev = NULL, *type = NULL;
 
     dev = (char *) PyUnicode_AsUTF8(_ped_Device_Type_obj.tp_repr(self->dev));
     if (dev == NULL) {
diff --git a/src/pyfilesys.c b/src/pyfilesys.c
index 53abd90..637a61d 100644
--- a/src/pyfilesys.c
+++ b/src/pyfilesys.c
@@ -171,14 +171,14 @@ PyObject *_ped_FileSystem_richcompare(_ped_FileSystem *a, PyObject *b, int op) {
 
 PyObject *_ped_FileSystem_str(_ped_FileSystem *self) {
     char *ret = NULL;
-    char *type = NULL, *geom = NULL;
+    const char *type = NULL, *geom = NULL;
 
-    type = (char *) PyUnicode_AsUTF8(_ped_FileSystem_Type_obj.tp_repr(self->type));
+    type = PyUnicode_AsUTF8(_ped_FileSystem_Type_obj.tp_repr(self->type));
     if (type == NULL) {
         return NULL;
     }
 
-    geom = (char *) PyUnicode_AsUTF8(_ped_Geometry_Type_obj.tp_repr(self->geom));
+    geom = PyUnicode_AsUTF8(_ped_Geometry_Type_obj.tp_repr(self->geom));
     if (geom == NULL) {
         return NULL;
     }
diff --git a/src/pygeom.c b/src/pygeom.c
index 811e6f5..a4263c0 100644
--- a/src/pygeom.c
+++ b/src/pygeom.c
@@ -86,9 +86,9 @@ PyObject *_ped_Geometry_richcompare(_ped_Geometry *a, PyObject *b, int op) {
 
 PyObject *_ped_Geometry_str(_ped_Geometry *self) {
     char *ret = NULL;
-    char *dev = NULL;
+    const char *dev = NULL;
 
-    dev = (char *) PyUnicode_AsUTF8(_ped_Device_Type_obj.tp_repr(self->dev));
+    dev = PyUnicode_AsUTF8(_ped_Device_Type_obj.tp_repr(self->dev));
     if (dev == NULL) {
         return NULL;
     }
diff --git a/src/pytimer.c b/src/pytimer.c
index 6cf1c95..11f8a8e 100644
--- a/src/pytimer.c
+++ b/src/pytimer.c
@@ -164,19 +164,21 @@ int _ped_Timer_set(_ped_Timer *self, PyObject *value, void *closure) {
             return -1;
         }
     } else if (!strcmp(member, "state_name")) {
-        self->state_name = (char *) PyUnicode_AsUTF8(value);
+        const char *state_name = PyUnicode_AsUTF8(value);
         if (PyErr_Occurred()) {
             return -1;
         }
         /* self->state_name now points to the internal buffer of a PyUnicode obj
          * which may be freed when its refcount drops to zero, so strdup it.
          */
-        if (self->state_name) {
-            self->state_name = strdup(self->state_name);
+        if (state_name) {
+            self->state_name = strdup(state_name);
             if (!self->state_name) {
                 PyErr_NoMemory();
                 return -2;
             }
+        } else {
+            self->state_name = NULL;
         }
     } else {
         PyErr_Format(PyExc_AttributeError, "_ped.Timer object has no attribute %s", member);
diff --git a/tests/baseclass.py b/tests/baseclass.py
index c599107..dc11d0c 100644
--- a/tests/baseclass.py
+++ b/tests/baseclass.py
@@ -32,7 +32,7 @@ def setUp(self):
         self.temp_prefix = "temp-device-"
         (self.fd, self.path) = tempfile.mkstemp(prefix=self.temp_prefix)
         self.f = os.fdopen(self.fd)
-        self.f.seek(140000)
+        self.f.seek(1400000)
         os.write(self.fd, b"0")
 
     def removeTempDevice(self):
@@ -68,11 +68,11 @@ def setUp(self):
         self.temp_prefix = "temp-device-"
         (self.fd, self.path,) = tempfile.mkstemp(prefix=self.temp_prefix)
         self.f = os.fdopen(self.fd)
-        self.f.seek(140000)
+        self.f.seek(1400000)
         os.write(self.fd, b"0")
         self.f.close()
 
-        os.system("mke2fs -F -q %s" % (self.path,))
+        assert(os.system("mke2fs -F -q %s" % (self.path,)) == 0)
 
         self._device = _ped.device_get(self.path)
         self._geometry = _ped.Geometry(self._device, 0, self._device.length - 1)
@@ -137,7 +137,7 @@ def closest(self, sector, a, b):
 class RequiresLabeledDevice(RequiresDevice):
     def setUp(self):
         RequiresDevice.setUp(self)
-        os.system("parted -s %s mklabel msdos" % (self.path,))
+        assert(os.system("parted -s %s mklabel msdos" % (self.path,)) == 0)
 
 # Base class for any test case that requires a _ped.Disk or parted.Disk.
 class RequiresDisk(RequiresDevice):
@@ -161,15 +161,15 @@ def setUp(self):
         self.mountpoint = None
 
     def mkfs(self):
-        os.system("mkfs.ext2 -F -q %s" % self.path)
+        assert(os.system("mkfs.ext2 -F -q %s" % self.path) == 0)
 
     def doMount(self):
         self.mountpoint = tempfile.mkdtemp()
-        os.system("mount -o loop %s %s" % (self.path, self.mountpoint))
+        assert(os.system("mount -o loop %s %s" % (self.path, self.mountpoint)) == 0)
 
     def removeMountpoint(self):
         if self.mountpoint and os.path.exists(self.mountpoint):
-            os.system("umount %s" % self.mountpoint)
+            assert(os.system("umount %s" % self.mountpoint) == 0)
             os.rmdir(self.mountpoint)
 
 # Base class for any test case that requires a _ped.Partition.
diff --git a/tests/test__ped_device.py b/tests/test__ped_device.py
index 33b250e..81384c7 100755
--- a/tests/test__ped_device.py
+++ b/tests/test__ped_device.py
@@ -243,7 +243,7 @@ def runTest(self):
 class UnitFormatCustomByteTestCase(RequiresDevice):
     def setUp(self):
         RequiresDevice.setUp(self)
-        pr = "%f" % (47.0 / self._device.unit_get_size(_ped.UNIT_PERCENT),)
+        pr = "%.2f" % (47.0 / self._device.unit_get_size(_ped.UNIT_PERCENT),)
         self.pairs = [(_ped.UNIT_SECTOR, '0s',),
                       (_ped.UNIT_BYTE, '47B',),
                       (_ped.UNIT_KILOBYTE, '0.05kB',),
@@ -253,7 +253,7 @@ def setUp(self):
                       (_ped.UNIT_COMPACT, '47.0B',),
                       (_ped.UNIT_CYLINDER, '0cyl',),
                       (_ped.UNIT_CHS, '0,0,0',),
-                      (_ped.UNIT_PERCENT, pr[:4] + "%",),
+                      (_ped.UNIT_PERCENT, pr + "%",),
                       (_ped.UNIT_KIBIBYTE, '0.05kiB',),
                       (_ped.UNIT_MEBIBYTE, '0.00MiB',),
                       (_ped.UNIT_GIBIBYTE, '0.00GiB',),
@@ -267,7 +267,7 @@ def runTest(self):
 class UnitFormatByteTestCase(RequiresDevice):
     def setUp(self):
         RequiresDevice.setUp(self)
-        pr = "%f" % (47.0 / self._device.unit_get_size(_ped.UNIT_PERCENT),)
+        pr = "%.2f" % (47.0 / self._device.unit_get_size(_ped.UNIT_PERCENT),)
         self._initialDefault = _ped.unit_get_default()
         self.pairs = [(_ped.UNIT_SECTOR, '0s',),
                       (_ped.UNIT_BYTE, '47B',),
@@ -278,7 +278,7 @@ def setUp(self):
                       (_ped.UNIT_COMPACT, '47.0B',),
                       (_ped.UNIT_CYLINDER, '0cyl',),
                       (_ped.UNIT_CHS, '0,0,0',),
-                      (_ped.UNIT_PERCENT, pr[:4] + "%",),
+                      (_ped.UNIT_PERCENT, pr + "%",),
                       (_ped.UNIT_KIBIBYTE, '0.05kiB',),
                       (_ped.UNIT_MEBIBYTE, '0.00MiB',),
                       (_ped.UNIT_GIBIBYTE, '0.00GiB',),
@@ -298,7 +298,7 @@ def setUp(self):
         RequiresDevice.setUp(self)
         sector_size = self._device.sector_size
         size = self._device.unit_get_size(_ped.UNIT_PERCENT)
-        pr = "%f" % ((47.0 * sector_size) / size,)
+        pr = "%.2f" % ((47.0 * sector_size) / size,)
         self.pairs = [(_ped.UNIT_SECTOR, '47s',),
                       (_ped.UNIT_BYTE, '24064B',),
                       (_ped.UNIT_KILOBYTE, '24.1kB',),
@@ -308,7 +308,7 @@ def setUp(self):
                       (_ped.UNIT_COMPACT, '24.1kB',),
                       (_ped.UNIT_CYLINDER, '0cyl',),
                       (_ped.UNIT_CHS, '0,1,15',),
-                      (_ped.UNIT_PERCENT, pr[:4] + "%",),
+                      (_ped.UNIT_PERCENT, pr + "%",),
                       (_ped.UNIT_KIBIBYTE, '23.5kiB',),
                       (_ped.UNIT_MEBIBYTE, '0.02MiB',),
                       (_ped.UNIT_GIBIBYTE, '0.00GiB',),
@@ -324,7 +324,7 @@ def setUp(self):
         RequiresDevice.setUp(self)
         sector_size = self._device.sector_size
         size = self._device.unit_get_size(_ped.UNIT_PERCENT)
-        pr = "%f" % ((47.0 * sector_size) / size,)
+        pr = "%.2f" % ((47.0 * sector_size) / size,)
         self._initialDefault = _ped.unit_get_default()
         self.pairs = [(_ped.UNIT_SECTOR, '47s',),
                       (_ped.UNIT_BYTE, '24064B',),
@@ -335,7 +335,7 @@ def setUp(self):
                       (_ped.UNIT_COMPACT, '24.1kB',),
                       (_ped.UNIT_CYLINDER, '0cyl',),
                       (_ped.UNIT_CHS, '0,1,15',),
-                      (_ped.UNIT_PERCENT, pr[:4] + "%",),
+                      (_ped.UNIT_PERCENT, pr + "%",),
                       (_ped.UNIT_KIBIBYTE, '23.5kiB',),
                       (_ped.UNIT_MEBIBYTE, '0.02MiB',),
                       (_ped.UNIT_GIBIBYTE, '0.00GiB',),