diff --git a/guppylang/checker/errors/type_errors.py b/guppylang/checker/errors/type_errors.py
index df7d771b..dddd94c7 100644
--- a/guppylang/checker/errors/type_errors.py
+++ b/guppylang/checker/errors/type_errors.py
@@ -74,7 +74,7 @@ class TypeInferenceError(Error):
 @dataclass(frozen=True)
 class IllegalConstant(Error):
     title: ClassVar[str] = "Unsupported constant"
-    span_label: ClassVar[str] = "Type `{ty}` is not supported"
+    span_label: ClassVar[str] = "Type `{python_ty.__name__}` is not supported"
     python_ty: type
 
 
diff --git a/tests/error/misc_errors/unsupported_const.err b/tests/error/misc_errors/unsupported_const.err
new file mode 100644
index 00000000..baf3536d
--- /dev/null
+++ b/tests/error/misc_errors/unsupported_const.err
@@ -0,0 +1,8 @@
+Error: Unsupported constant (at $FILE:7:8)
+  | 
+5 | @compile_guppy
+6 | def foo() -> None:
+7 |     x = "foo"
+  |         ^^^^^ Type `str` is not supported
+
+Guppy compilation failed due to 1 previous error
diff --git a/tests/error/misc_errors/unsupported_const.py b/tests/error/misc_errors/unsupported_const.py
new file mode 100644
index 00000000..1959d05b
--- /dev/null
+++ b/tests/error/misc_errors/unsupported_const.py
@@ -0,0 +1,7 @@
+from guppylang.std.builtins import array
+from tests.util import compile_guppy
+
+
+@compile_guppy
+def foo() -> None:
+    x = "foo"