Add test for neggative gamma parameter

skimage/exposure/exposure.py

@@ -263,7 +263,7 @@ def adjust_gamma(image, gamma=1, gain=1):
     dtype = image.dtype.type
 
     if gamma < 0:
-        return "Gamma should be a non-negative real number"
+        raise ValueError("Gamma should be a non-negative real number.")
 
     scale = float(dtype_limits(image, True)[1] - dtype_limits(image, True)[0])
 

skimage/exposure/tests/test_exposure.py

@@ -230,6 +230,11 @@ def test_adjust_gamma_greater_one():
     assert_array_equal(result, expected)
 
 
+def test_adjust_gamma_neggative():
+    image = np.arange(0, 255, 4, np.uint8).reshape(8,8)
+    assert_raises(ValueError, exposure.adjust_gamma, image, -1)
+
+
 # Test Logarithmic Correction
 # ===========================