diff --git a/lib/matplotlib/cm.py b/lib/matplotlib/cm.py
index dcc0a1db6bd..f492bb430c1 100644
--- a/lib/matplotlib/cm.py
+++ b/lib/matplotlib/cm.py
@@ -221,6 +221,9 @@ def to_rgba(self, x, alpha=None, bytes=False, norm=True):
         If *x* is an ndarray with 3 dimensions,
         and the last dimension is either 3 or 4, then it will be
         treated as an rgb or rgba array, and no mapping will be done.
+        The array can be uint8, or it can be floating point with
+        values in the 0-1 range; otherwise a ValueError will be raised.
+        If it is a masked array, the mask will be ignored.
         If the last dimension is 3, the *alpha* kwarg (defaulting to 1)
         will be used to fill in the transparency.  If the last dimension
         is 4, the *alpha* kwarg is ignored; it does not
@@ -232,12 +235,8 @@ def to_rgba(self, x, alpha=None, bytes=False, norm=True):
         the returned rgba array will be uint8 in the 0 to 255 range.
 
         If norm is False, no normalization of the input data is
-        performed, and it is assumed to already be in the range (0-1).
+        performed, and it is assumed to be in the range (0-1).
 
-        Note: this method assumes the input is well-behaved; it does
-        not check for anomalies such as *x* being a masked rgba
-        array, or being an integer type other than uint8, or being
-        a floating point rgba array with values outside the 0-1 range.
         """
         # First check for special case, image input:
         try:
@@ -255,10 +254,18 @@ def to_rgba(self, x, alpha=None, bytes=False, norm=True):
                     xx = x
                 else:
                     raise ValueError("third dimension must be 3 or 4")
-                if bytes and xx.dtype != np.uint8:
-                    xx = (xx * 255).astype(np.uint8)
-                if not bytes and xx.dtype == np.uint8:
-                    xx = xx.astype(float) / 255
+                if xx.dtype.kind == 'f':
+                    if xx.max() > 1 or xx.min() < 0:
+                        raise ValueError("Floating point image RGB values "
+                                         "must be in the 0..1 range.")
+                    if bytes:
+                        xx = (xx * 255).astype(np.uint8)
+                elif xx.dtype == np.uint8:
+                    if not bytes:
+                        xx = xx.astype(float) / 255
+                else:
+                    raise ValueError("Image RGB array must be uint8 or "
+                                     "floating point; found %s" % xx.dtype)
                 return xx
         except AttributeError:
             # e.g., x is not an ndarray; so try mapping it