diff --git a/eval.c b/eval.c
index 60b7ade..507e773 100644
--- a/eval.c
+++ b/eval.c
@@ -5,6 +5,9 @@
 #include "util.h"
 #include "internals.h"
 
+int currentRecursionDepth = 0;
+#define MAXRECURSIONDEPTH 1000
+
 object apply(object function, object parameters, env currentEnv);
 
 object eval(object input, env currentEnv)
@@ -97,6 +100,11 @@ object apply(object procedure, object parameters, env currentEnv)
 		return result;
 	}
 
+	if (++currentRecursionDepth > MAXRECURSIONDEPTH)
+	{
+		--currentRecursionDepth;
+		SIGERR(maxRecursionDepthError);
+	}
 	object args = copyObject(PROC_COMP_ARGS(procedure));
 	object body = copyObject(PROC_COMP_BODY(procedure));
 	env definitionEnv = PROC_COMP_ENV(procedure);
@@ -123,13 +131,14 @@ object apply(object procedure, object parameters, env currentEnv)
 		CAR(*currentSubProc) = eval(CAR(*currentSubProc), procEnv);
 		if (TYPE(CDR(*currentSubProc)) == nilObject)
 		{
-			result = CAR(*currentSubProc);
+			result = copyObject(CAR(*currentSubProc));
 		}
 		currentSubProc = &CDR(*currentSubProc);
 	}
 	deleteObject(args);
 	deleteObject(body);
 	removeEnvironment(procEnv);
+	--currentRecursionDepth;
 
 	return result;
 }