diff --git a/msp/test.py b/msp/test.py
index 14cd322..7d12012 100755
--- a/msp/test.py
+++ b/msp/test.py
@@ -18,7 +18,7 @@
 ]
 
 
-def test(f):
+def test(f: str) -> None:
     try:
         im = Image.open(f)
         im.show()
diff --git a/picins/test.py b/picins/test.py
index 32f27f2..5cb9487 100755
--- a/picins/test.py
+++ b/picins/test.py
@@ -7,7 +7,7 @@
 files = ["mandel.msp", "mexhat1.msp", "mexhat2.msp"]
 
 
-def test(f):
+def test(f: str) -> None:
     try:
         im = Image.open(f)
         im.show()
diff --git a/sunraster/test.py b/sunraster/test.py
index 42c97be..53df0f8 100755
--- a/sunraster/test.py
+++ b/sunraster/test.py
@@ -18,7 +18,7 @@
 ]
 
 
-def test(f):
+def test(f: str) -> None:
     try:
         im = Image.open(f)
         im.show()