diff --git a/pdfminer/pdfdocument.py b/pdfminer/pdfdocument.py
index 9192262..5dd2b9e 100644
--- a/pdfminer/pdfdocument.py
+++ b/pdfminer/pdfdocument.py
@@ -190,6 +190,7 @@ class PDFXRefFallback(PDFXRef):
                         objs.append(obj)
                 except PSEOF:
                     pass
+                n = min(n, len(objs)/2)
                 for index in xrange(n):
                     objid1 = objs[index*2]
                     self.offsets[objid1] = (objid, index, 0)
diff --git a/pdfminer/pdfinterp.py b/pdfminer/pdfinterp.py
index 27745fb..788bf8b 100644
--- a/pdfminer/pdfinterp.py
+++ b/pdfminer/pdfinterp.py
@@ -554,12 +554,20 @@ class PDFPageInterpreter(object):
 
     # setcolorspace-stroking
     def do_CS(self, name):
-        self.scs = self.csmap[literal_name(name)]
+        try:
+            self.scs = self.csmap[literal_name(name)]
+        except KeyError:
+            if STRICT:
+                raise PDFInterpreterError('Undefined ColorSpace: %r' % name)
         return
 
     # setcolorspace-non-strokine
     def do_cs(self, name):
-        self.ncs = self.csmap[literal_name(name)]
+        try:
+            self.ncs = self.csmap[literal_name(name)]
+        except KeyError:
+            if STRICT:
+                raise PDFInterpreterError('Undefined ColorSpace: %r' % name)
         return
 
     # setgray-stroking