diff --git a/bob/io/base/__init__.py b/bob/io/base/__init__.py
index 7d4d6f5e6203a35aa4766aa9d6232b092b75b8d8..ea35d2c3b70cc88a4cde18cf793f04871ddcdf1b 100644
--- a/bob/io/base/__init__.py
+++ b/bob/io/base/__init__.py
@@ -116,7 +116,7 @@ def open_file(filename):
         img = check_gray(img)
         return img if img.ndim == 2 else to_bob(img)
     else:
-        raise RuntimeError(f"Unknown file extension: {extension}")
+        raise ValueError(f"Unknown file extension: {extension}")
 
 
 def write_file(filename, data, format="pillow"):