diff --git a/skimage/util/dtype.py b/skimage/util/dtype.py
index 3340434a..ab56b98e 100644
--- a/skimage/util/dtype.py
+++ b/skimage/util/dtype.py
@@ -161,7 +161,7 @@ def convert(image, dtype, force_copy=False, uniform=False, frange=[-1.5, 1.5]):
 
     if kind_in == 'f':
         if frange and np.min(image) < frange[0] or np.max(image) > frange[1]:
-            raise ValueError("Images of type float must be between %d and %d",
+            raise ValueError("Images of type float must be between %g and %g",
                              tuple(frange))
         if kind == 'f':
             # floating point -> floating point