diff --git a/llvm_emulator/stepper.py b/llvm_emulator/stepper.py
index 573bf05..61cf031 100644
--- a/llvm_emulator/stepper.py
+++ b/llvm_emulator/stepper.py
@@ -132,7 +132,7 @@ def step(insns, terminator, blocks, stack_frames, ssa_env, global_env, heap,
             function = fdecs[callee.val]
         except KeyError:
             err('Could not find function {} in environment:\n{}'
-                .format(callee.val, fdecs.keys()))
+                .format(callee.val, list(fdecs.keys())))
             return insns_rest, terminator, blocks, stack_frames, ssa_env, heap, call_res
 
         parameters = function.parameters