diff --git a/core/src/test/scala/eu/neverblink/jelly/core/ProtoAuxiliarySpec.scala b/core/src/test/scala/eu/neverblink/jelly/core/ProtoAuxiliarySpec.scala
index 6dda1d9b9..059f63bdd 100644
--- a/core/src/test/scala/eu/neverblink/jelly/core/ProtoAuxiliarySpec.scala
+++ b/core/src/test/scala/eu/neverblink/jelly/core/ProtoAuxiliarySpec.scala
@@ -101,6 +101,20 @@ class ProtoAuxiliarySpec extends AnyWordSpec, Matchers:
       }
       exception.getMessage should include("depth exceeded: 65")
     }
+
+    "not preserve the cached size on clone()" in {
+      // .clone() should not preserve the cached size, as the cloned object will likely be modified
+      // right after cloning.
+      // Cloning while preserving the cached size leads to weird, hard-to-debug issues, for example:
+      // https://github.com/Jelly-RDF/cli/pull/126
+      val frame = RdfStreamFrame.newInstance()
+        .addRows(RdfStreamRow.newInstance().setTriple(RdfTriple.newInstance()))
+      val s = frame.getSerializedSize
+      s should be > 0
+      frame.getCachedSize should be (s)
+      val clonedFrame = frame.clone()
+      clonedFrame.getCachedSize should be(-1)
+    }
   }
 
   // Tests for the core-protos-google module
diff --git a/crunchy-protoc-plugin/src/main/scala/eu/neverblink/protoc/java/gen/MessageGenerator.scala b/crunchy-protoc-plugin/src/main/scala/eu/neverblink/protoc/java/gen/MessageGenerator.scala
index de4980086..bcff72dad 100644
--- a/crunchy-protoc-plugin/src/main/scala/eu/neverblink/protoc/java/gen/MessageGenerator.scala
+++ b/crunchy-protoc-plugin/src/main/scala/eu/neverblink/protoc/java/gen/MessageGenerator.scala
@@ -375,7 +375,11 @@ class MessageGenerator(val info: MessageInfo):
       .addParameter(info.typeName, "other", Modifier.FINAL)
       .addModifiers(Modifier.PUBLIC, Modifier.FINAL)
       .returns(info.mutableTypeName)
-    copyFrom.addStatement("cachedSize = other.cachedSize")
+    // Reset cached size -- definitely do not copy it from the other message, because this object
+    // will likely be modified right after copying, leading to an incorrect cached size.
+    // Cloning while preserving the cached size leads to weird, hard-to-debug issues, for example:
+    // https://github.com/Jelly-RDF/cli/pull/126
+    copyFrom.addStatement("cachedSize = -1")
     fields.foreach(_.generateCopyFromCode(copyFrom))
     oneOfGenerators.foreach(_.generateCopyFromCode(copyFrom))
     copyFrom.addStatement("return this")