=== modified file 'exif.py'
--- exif.py	2011-09-06 13:19:24 +0000
+++ exif.py	2011-09-06 13:16:29 +0000
@@ -426,7 +426,14 @@
                 raise ExifValueError(value, self.type)
 
         elif self.type == 'Rational':
-            if is_fraction(value) and value.numerator >= 0:
+            if isinstance(value, unicode):
+                try:
+                    return value.encode('utf-8')
+                except UnicodeEncodeError:
+                    raise ExifValueError(value, self.type)
+            elif isinstance(value, str):
+                return value
+            elif is_fraction(value) and value.numerator >= 0:
                 return fraction_to_string(value)
             else:
                 raise ExifValueError(value, self.type)

=== modified file 'utils.py'
--- utils.py	2011-09-06 13:19:24 +0000
+++ utils.py	2011-09-06 13:16:29 +0000
@@ -282,10 +282,18 @@
     """
     Test whether the object is a valid fraction.
     """
-    if Fraction is not None and isinstance(obj, Fraction):
-        return True
+    if Fraction is not None:
+        if isinstance(obj, Fraction):
+            return True
+        obj = Fraction(obj)
+        if (obj.denomenator > 0):
+            return True
     elif isinstance(obj, Rational):
         return True
+    elif isinstance(obj, str):
+        obj = Rational.fromstring(obj)
+        if (obj.denomenator > 0):
+            return True
     else:
         return False