diff --git a/ext/mbstring/libmbfl/filters/mbfilter_7bit.c b/ext/mbstring/libmbfl/filters/mbfilter_7bit.c
index a3a90c96f7a..2885ca1fbd6 100644
--- a/ext/mbstring/libmbfl/filters/mbfilter_7bit.c
+++ b/ext/mbstring/libmbfl/filters/mbfilter_7bit.c
@@ -76,7 +76,7 @@ int mbfl_filt_conv_any_7bit(int c, mbfl_convert_filter *filter)
 	if (c >= 0 && c < 0x80) {
 		CK((*filter->output_function)(c, filter->data));
 	} else {
-		mbfl_filt_conv_illegal_output(c, filter);
+		CK(mbfl_filt_conv_illegal_output(c, filter));
 	}
 	return 0;
 }
diff --git a/ext/mbstring/libmbfl/filters/mbfilter_sjis_mobile.c b/ext/mbstring/libmbfl/filters/mbfilter_sjis_mobile.c
index d023a4940a9..cc04335fcc4 100644
--- a/ext/mbstring/libmbfl/filters/mbfilter_sjis_mobile.c
+++ b/ext/mbstring/libmbfl/filters/mbfilter_sjis_mobile.c
@@ -462,7 +462,7 @@ int mbfilter_unicode2sjis_emoji_kddi(int c, int *s1, mbfl_convert_filter *filter
 
 		/* If none of the KDDI national flag emoji matched, then we have no way
 		 * to convert the previous codepoint... */
-		mbfl_filt_conv_illegal_output(c1, filter);
+		CK(mbfl_filt_conv_illegal_output(c1, filter));
 	}
 
 	if (c == '#' || (c >= '0' && c <= '9')) {
@@ -534,7 +534,7 @@ int mbfilter_unicode2sjis_emoji_sb(int c, int *s1, mbfl_convert_filter *filter)
 
 		/* If none of the SoftBank national flag emoji matched, then we have no way
 		 * to convert the previous codepoint... */
-		mbfl_filt_conv_illegal_output(c1, filter);
+		CK(mbfl_filt_conv_illegal_output(c1, filter));
 	}
 
 	if (c == '#' || (c >= '0' && c <= '9')) {
@@ -855,7 +855,7 @@ int mbfl_filt_conv_sjis_mobile_flush(mbfl_convert_filter *filter)
 	} else if (filter->status == 2) {
 		/* First of a pair of Regional Indicator codepoints came at the end of a string */
 		filter->cache = filter->status = 0;
-		mbfl_filt_conv_illegal_output(c1, filter);
+		CK(mbfl_filt_conv_illegal_output(c1, filter));
 	}
 
 	if (filter->flush_function) {