diff --git a/libtvm/tvm_preprocessor.c b/libtvm/tvm_preprocessor.c
index 347da51..82286ee 100644
--- a/libtvm/tvm_preprocessor.c
+++ b/libtvm/tvm_preprocessor.c
@@ -22,7 +22,7 @@ int tvm_preprocess(char *src, int *src_len, tvm_tree_t **node)
 		if(!pFile)
 		{
 			printf("Unable to open file \"%s\"\n", filename);
-			return 0;
+			return -1;
 		}
 
 		free(temp_str);
@@ -60,7 +60,7 @@ int tvm_preprocess(char *src, int *src_len, tvm_tree_t **node)
 		if(begin + offset >= end)
 		{
 			printf("Define missing arguments.\n");
-			return 0;
+			return -1;
 		}
 
 		int length = (end - (begin + offset));
@@ -82,7 +82,7 @@ int tvm_preprocess(char *src, int *src_len, tvm_tree_t **node)
 		if(!keystr || !valstr)
 		{
 			printf("Define missing arguments.\n");
-			return 0;
+			return -1;
 		}
 
 		int err = 0;
@@ -94,7 +94,7 @@ int tvm_preprocess(char *src, int *src_len, tvm_tree_t **node)
 		if(err == 2)
 		{
 			printf("Multiple definitions for %s.\n", keystr);
-			return 0;
+			return -1;
 		}
 
 		/* Remove the define line so it is not processed again. */
diff --git a/libtvm/tvm_program.c b/libtvm/tvm_program.c
index 7003591..ca74421 100644
--- a/libtvm/tvm_program.c
+++ b/libtvm/tvm_program.c
@@ -54,7 +54,12 @@ pi_interpret:
 	tvm_fcopy(source, source_length, pFile);
 	fclose(pFile);
 
-	while(tvm_preprocess(source, &source_length, &p->defines));
+	int err = 0;
+	while((err = tvm_preprocess(source, &source_length, &p->defines)) > 0);
+
+	/* The preprocessor encountered a problem. */
+	if (err < 0)
+		return 1;
 
 	tvm_lexer_t *lexer_ctx = lexer_create();
 	lex(lexer_ctx, source, &p->defines);