diff --git a/Makefile b/Makefile
index ad186c2..dc07094 100644
--- a/Makefile
+++ b/Makefile
@@ -39,6 +39,9 @@ venv/.done: requirements.txt
 	venv/bin/python3 -m pip install -r $^
 	touch $@
 
+clean-examples:
+	rm -f examples/*.wat examples/*.wasm examples/*.wat.html examples/*.py.html
+
 .SECONDARY: # Keep intermediate files
 
 .PHONY: examples
diff --git a/phasm/codestyle.py b/phasm/codestyle.py
index f1031ad..48b9606 100644
--- a/phasm/codestyle.py
+++ b/phasm/codestyle.py
@@ -23,6 +23,9 @@ def type3(inp: Type3OrPlaceholder) -> str:
     """
     assert isinstance(inp, Type3), TYPE3_ASSERTION_ERROR
 
+    if inp is type3types.none:
+        return 'None'
+
     if isinstance(inp, type3types.AppliedType3):
         if inp.base == type3types.tuple:
             return '(' + ', '.join(
diff --git a/phasm/compiler.py b/phasm/compiler.py
index 5ba2793..a511d0e 100644
--- a/phasm/compiler.py
+++ b/phasm/compiler.py
@@ -40,6 +40,9 @@ def type3(inp: type3types.Type3OrPlaceholder) -> wasm.WasmType:
     """
     assert isinstance(inp, type3types.Type3), type3types.TYPE3_ASSERTION_ERROR
 
+    if inp == type3types.none:
+        return wasm.WasmTypeNone()
+
     if inp == type3types.u8:
         # WebAssembly has only support for 32 and 64 bits
         # So we need to store more memory per byte
diff --git a/phasm/type3/constraintsgenerator.py b/phasm/type3/constraintsgenerator.py
index 241d0ec..2093303 100644
--- a/phasm/type3/constraintsgenerator.py
+++ b/phasm/type3/constraintsgenerator.py
@@ -115,6 +115,8 @@ def expression(ctx: Context, inp: ourlang.Expression) -> Generator[ConstraintBas
     raise NotImplementedError(expression, inp)
 
 def function(ctx: Context, inp: ourlang.Function) -> Generator[ConstraintBase, None, None]:
+    assert not inp.imported
+
     if isinstance(inp, ourlang.StructConstructor):
         return
 
@@ -136,4 +138,7 @@ def module(ctx: Context, inp: ourlang.Module) -> Generator[ConstraintBase, None,
         yield from module_constant_def(ctx, cdef)
 
     for func in inp.functions.values():
+        if func.imported:
+            continue
+
         yield from function(ctx, func)
diff --git a/tests/integration/test_lang/test_interface.py b/tests/integration/test_lang/test_interface.py
index cbacd73..a8dfc32 100644
--- a/tests/integration/test_lang/test_interface.py
+++ b/tests/integration/test_lang/test_interface.py
@@ -1,9 +1,11 @@
 import pytest
 
+from phasm.type3.entry import Type3Exception
+
 from ..helpers import Suite
 
 @pytest.mark.integration_test
-def test_imported():
+def test_imported_ok():
     code_py = """
 @imported
 def helper(mul: i32) -> i32:
@@ -25,3 +27,53 @@ def testEntry() -> i32:
     )
 
     assert 8476 == result.returned_value
+
+@pytest.mark.integration_test
+def test_imported_side_effect_no_return():
+    code_py = """
+@imported
+def helper(mul: u8) -> None:
+    pass
+
+@exported
+def testEntry() -> None:
+    return helper(3)
+"""
+    prop = None
+
+    def helper(mul: int) -> None:
+        nonlocal prop
+        prop = mul
+
+    result = Suite(code_py).run_code(
+        runtime='wasmer',
+        imports={
+            'helper': helper,
+        }
+    )
+
+    assert None is result.returned_value
+    assert 3 == prop
+
+@pytest.mark.integration_test
+def test_imported_type_mismatch():
+    code_py = """
+@imported
+def helper(mul: u8) -> u8:
+    pass
+
+@exported
+def testEntry(x: u32) -> u8:
+    return helper(x)
+"""
+
+    def helper(mul: int) -> int:
+        return 4238 * mul
+
+    with pytest.raises(Type3Exception, match=r'u32 must be u8 instead'):
+        Suite(code_py).run_code(
+            runtime='wasmer',
+            imports={
+                'helper': helper,
+            }
+        )