diff --git a/asterix-runtime/src/main/java/edu/uci/ics/asterix/runtime/evaluators/common/FunctionManagerImpl.java b/asterix-runtime/src/main/java/edu/uci/ics/asterix/runtime/evaluators/common/FunctionManagerImpl.java
index 6f834a7..127d4a6 100644
--- a/asterix-runtime/src/main/java/edu/uci/ics/asterix/runtime/evaluators/common/FunctionManagerImpl.java
+++ b/asterix-runtime/src/main/java/edu/uci/ics/asterix/runtime/evaluators/common/FunctionManagerImpl.java
@@ -37,7 +37,11 @@
     @Override
     public synchronized IFunctionDescriptor lookupFunction(FunctionIdentifier fid) throws AlgebricksException {
         Pair<FunctionIdentifier, Integer> key = new Pair<FunctionIdentifier, Integer>(fid, fid.getArity());
-        return functions.get(key).createFunctionDescriptor();
+        IFunctionDescriptorFactory factory = functions.get(key);
+        if (factory == null) {
+            throw new AlgebricksException("Inappropriate use of function " + "'" + fid.getName() + "'");
+        }
+        return factory.createFunctionDescriptor();
     }
 
     @Override