diff --git a/skimage/util/dtype.py b/skimage/util/dtype.py
index ed1ae35b..44476e44 100644
--- a/skimage/util/dtype.py
+++ b/skimage/util/dtype.py
@@ -31,8 +31,9 @@ def convert(image, dtype, force_copy=False, uniform=False):
     Warnings are issued in case of precision loss, or when negative values
     are clipped during conversion to unsigned integer types (sign loss).
 
-    Floating point values will be clipped to the range [0.0, 1.0] or
-    [-1.0, 1.0] when converting to unsigned or signed integers respectively.
+    Floating point values are expected to be normalized and will be clipped
+    to the range [0.0, 1.0] or [-1.0, 1.0] when converting to unsigned or
+    signed integers respectively.
 
     Numbers are not shifted to the negative side when converting from
     unsigned to signed integer types. Negative values will be clipped when
@@ -152,6 +153,8 @@ def convert(image, dtype, force_copy=False, uniform=False):
         imax_in = np.iinfo(dtype_in).max
 
     if kind_in == 'f':
+        if np.min(image) < -1.0 or np.max(image) > 1.0:
+            raise ValueError("Images of type float must be between -1 and 1.")
         if kind == 'f':
             # floating point -> floating point
             if itemsize_in > itemsize: