diff --git a/main.c b/main.c
index 79b8cc4..ab78d10 100644
--- a/main.c
+++ b/main.c
@@ -78,55 +78,59 @@ static void error(const char *msg, ...)
 	va_end(params);
 }
 
-static int process_input(const char *input, char **out, unsigned int flags)
+static int velthuis_encode(const char *in, char **out, unsigned int flags)
 {
 	char *tmp = NULL;
 	int ret;
 
-	if (flags & FLAG_REVERSE) {
-		if (flags & FLAG_VELTHUIS) {
-			return encode_iast_to_velthuis(input, out);
-		} else {
-			return transliterate_latin_to_devanagari(input, out);
-		}
+	if (flags & FLAG_REVERSE)
+		return encode_iast_to_velthuis(in, out);
+
+	ret = encode_velthuis_to_iast(in, out);
+	if (flags & FLAG_DEVANAGARI) {
+		ret = transliterate_latin_to_devanagari(*out, &tmp);
+		free(*out);
+		*out = tmp;
 	}
 
-	if (flags & FLAG_VELTHUIS) {
-		if (flags & FLAG_DEVANAGARI) {
-			ret = encode_velthuis_to_iast(input, &tmp);
-			if (ret != 0)
-				return ret;
+	return ret;
+}
 
-			ret = transliterate_latin_to_devanagari(tmp, out);
-			free(tmp);
-			return ret;
-		} else {
-			return encode_velthuis_to_iast(input, out);
-		}
+static int iast_transliterate(const char *in, char **out, unsigned int flags)
+{
+	char *tmp = NULL;
+	int ret;
+
+	if (flags & FLAG_REVERSE)
+		return transliterate_latin_to_devanagari(in, out);
+
+	ret = transliterate_devanagari_to_latin(in, out);
+	if (flags & FLAG_ASCII) {
+		ret = encode_iast_to_velthuis(*out, &tmp);
+		free(*out);
+		*out = tmp;
 	}
 
+	return ret;
+}
+
+static int process_input(const char *input, char **out, unsigned int flags)
+{
+	if (flags & FLAG_HINDI)
+		return transcript_devanagari_to_hindi(input, out);
+
 	if (flags & FLAG_CZECH)
 		return transcript_devanagari_to_czech(input, out);
 
-	if (flags & FLAG_HINDI)
-		return transcript_devanagari_to_hindi(input, out);
+	if (flags & FLAG_VELTHUIS)
+		return velthuis_encode(input, out, flags);
 
-	if (flags & FLAG_ASCII) {
-		ret = transliterate_devanagari_to_latin(input, &tmp);
-		if (ret != 0)
-			return ret;
-
-		ret = encode_iast_to_velthuis(tmp, out);
-		free(tmp);
-		return ret;
-	} else {
-		return transliterate_devanagari_to_latin(input, out);
-	}
+	return iast_transliterate(input, out, flags);
 }
 
 static int process_string(const char *input, unsigned int flags)
 {
-	char *output;
+	char *output = NULL;
 	int ret;
 
 	ret = process_input(input, &output, flags);
@@ -145,6 +149,21 @@ static int process_string(const char *input, unsigned int flags)
 	return ret;
 }
 
+static int validate_flags(unsigned int flags)
+{
+	if (flags & FLAG_HINDI && flags & FLAG_REVERSE) {
+		error("invalid combination of '-H' and '-r'.");
+		return -EINVAL;
+	}
+
+	if (flags & FLAG_CZECH && flags & FLAG_REVERSE) {
+		error("invalid combination of '-c' and '-r'.");
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
 #define CHUNKSIZE 1024
 static int read_fd(char **out, int fd)
 {
@@ -247,6 +266,10 @@ int main(int argc, const char **argv)
 		}
 	}
 
+	retval = validate_flags(flags);
+	if (retval != 0)
+		return retval;
+
 	while (optind < argc) {
 		const char *arg = argv[optind++];
 
diff --git a/tests/integration.c b/tests/integration.c
index df3bcba..50c032f 100644
--- a/tests/integration.c
+++ b/tests/integration.c
@@ -148,6 +148,8 @@ START_TEST(test_errors)
 {
 	test_output("./iast -x 2>&1", "[iast] error: unrecognised option '-x'.\n");
 	test_output("./iast -f xxx 2>&1", "[iast] error: failed to read file 'xxx'.\n");
+	test_output("./iast -Hr 2>&1", "[iast] error: invalid combination of '-H' and '-r'.\n");
+	test_output("./iast -cr 2>&1", "[iast] error: invalid combination of '-c' and '-r'.\n");
 }
 END_TEST