diff --git a/guppylang/checker/func_checker.py b/guppylang/checker/func_checker.py
index 7463cf7b..c684aac3 100644
--- a/guppylang/checker/func_checker.py
+++ b/guppylang/checker/func_checker.py
@@ -176,6 +176,10 @@ def check_signature(func_def: ast.FunctionDef, globals: Globals) -> FunctionType
         raise GuppyError(UnsupportedError(func_def.args.vararg, "Variadic args"))
     if func_def.args.kwarg is not None:
         raise GuppyError(UnsupportedError(func_def.args.kwarg, "Keyword args"))
+    if func_def.args.defaults:
+        raise GuppyError(
+            UnsupportedError(func_def.args.defaults[0], "Default arguments")
+        )
     if func_def.returns is None:
         err = MissingReturnAnnotationError(func_def)
         # TODO: Error location is incorrect
diff --git a/tests/error/misc_errors/default_arg.err b/tests/error/misc_errors/default_arg.err
new file mode 100644
index 00000000..9de36146
--- /dev/null
+++ b/tests/error/misc_errors/default_arg.err
@@ -0,0 +1,8 @@
+Error: Unsupported (at $FILE:5:26)
+  | 
+3 | 
+4 | @compile_guppy
+5 | def foo(x: int, y: bool = True) -> int:
+  |                           ^^^^ Default arguments are not supported
+
+Guppy compilation failed due to 1 previous error
diff --git a/tests/error/misc_errors/default_arg.py b/tests/error/misc_errors/default_arg.py
new file mode 100644
index 00000000..a4eacc97
--- /dev/null
+++ b/tests/error/misc_errors/default_arg.py
@@ -0,0 +1,6 @@
+from tests.util import compile_guppy
+
+
+@compile_guppy
+def foo(x: int, y: bool = True) -> int:
+    return x