diff --git a/Makefile.am b/Makefile.am
index 6439cf9..9c0df2e 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -16,6 +16,8 @@ seq2gif_CFLAGS = $(MAYBE_COVERAGE)
 
 test: all
 	./seq2gif -i tests/data/sl.tty -o /dev/null
+	! ./seq2gif -i /non-existent-file.tty -o /dev/null
+	! ./seq2gif -i tests/data/sl.tty -o /non-existent-file.out
 
 coveralls: test
 	coveralls -E '.*\.h' -e tests -e glyph -e m4
diff --git a/Makefile.in b/Makefile.in
index 85625a7..9f58958 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -906,6 +906,8 @@ uninstall-am: uninstall-binPROGRAMS
 
 test: all
 	./seq2gif -i tests/data/sl.tty -o /dev/null
+	! ./seq2gif -i /non-existent-file.tty -o /dev/null
+	! ./seq2gif -i tests/data/sl.tty -o /non-existent-file.out
 
 coveralls: test
 	coveralls -E '.*\.h' -e tests -e glyph -e m4
diff --git a/main.c b/main.c
index 91474c0..ae3f7d6 100644
--- a/main.c
+++ b/main.c
@@ -565,6 +565,9 @@ int main(int argc, char *argv[])
 
     in_file = open_input_file(settings.input);
     out_file = open_output_file(settings.output);
+    if (in_file == NULL || out_file == NULL) {
+        exit(1);
+    }
 
     maxlen = 2048;
     obuf = malloc(maxlen);