diff --git a/bio2zarr/vcf2zarr/icf.py b/bio2zarr/vcf2zarr/icf.py
index 0559c340..c060af78 100644
--- a/bio2zarr/vcf2zarr/icf.py
+++ b/bio2zarr/vcf2zarr/icf.py
@@ -96,7 +96,7 @@ def smallest_dtype(self):
         elif self.vcf_type == "Flag":
             ret = "bool"
         elif self.vcf_type == "Character":
-            ret = "U1"
+            ret = "S1"
         else:
             assert self.vcf_type == "String"
             ret = "O"
diff --git a/tests/test_icf.py b/tests/test_icf.py
index c9a79425..78b4a4fc 100644
--- a/tests/test_icf.py
+++ b/tests/test_icf.py
@@ -217,12 +217,12 @@ def schema(self, icf):
             ("variant_IFA", "f4", (208, 2), ("variants", "alt_alleles")),
             ("variant_IFR", "f4", (208, 3), ("variants", "alleles")),
             ("variant_IFD", "f4", (208, 9), ("variants", "INFO_IFD_dim")),
-            ("variant_IC1", "U1", (208,), ("variants",)),
-            ("variant_IC2", "U1", (208, 2), ("variants", "INFO_IC2_dim")),
+            ("variant_IC1", "S1", (208,), ("variants",)),
+            ("variant_IC2", "S1", (208, 2), ("variants", "INFO_IC2_dim")),
             ("variant_IS1", "O", (208,), ("variants",)),
             ("variant_IS2", "O", (208, 2), ("variants", "INFO_IS2_dim")),
             ("call_FS2", "O", (208, 2, 2), ("variants", "samples", "FORMAT_FS2_dim")),
-            ("call_FC2", "U1", (208, 2, 2), ("variants", "samples", "FORMAT_FC2_dim")),
+            ("call_FC2", "S1", (208, 2, 2), ("variants", "samples", "FORMAT_FC2_dim")),
             ("call_FIG", "i2", (208, 2, 6), ("variants", "samples", "genotypes")),
             ("call_FIA", "i2", (208, 2, 2), ("variants", "samples", "alt_alleles")),
             ("call_FIR", "i2", (208, 2, 3), ("variants", "samples", "alleles")),