diff --git a/ext/mbstring/libmbfl/filters/mbfilter_utf8.c b/ext/mbstring/libmbfl/filters/mbfilter_utf8.c
index 6c7bad0e805..46bddd17a73 100644
--- a/ext/mbstring/libmbfl/filters/mbfilter_utf8.c
+++ b/ext/mbstring/libmbfl/filters/mbfilter_utf8.c
@@ -249,11 +249,9 @@ static size_t mb_utf8_to_wchar(unsigned char **in, size_t *in_len, uint32_t *buf
 					p--;
 				} else {
 					uint32_t decoded = ((c & 0xF) << 12) | ((c2 & 0x3F) << 6) | (c3 & 0x3F);
-					if (decoded < 0x800 || (decoded >= 0xD800 && decoded <= 0xDFFF)) {
-						*out++ = MBFL_BAD_INPUT;
-					} else {
-						*out++ = decoded;
-					}
+					ZEND_ASSERT(decoded >= 0x800); /* Not an overlong code unit */
+					ZEND_ASSERT(decoded < 0xD800 || decoded > 0xDFFF); /* U+D800-DFFF are reserved, illegal code points */
+					*out++ = decoded;
 				}
 			} else {
 				*out++ = MBFL_BAD_INPUT;
@@ -283,7 +281,8 @@ static size_t mb_utf8_to_wchar(unsigned char **in, size_t *in_len, uint32_t *buf
 					p--;
 				} else {
 					uint32_t decoded = ((c & 0x7) << 18) | ((c2 & 0x3F) << 12) | ((c3 & 0x3F) << 6) | (c4 & 0x3F);
-					*out++ = (decoded < 0x10000) ? MBFL_BAD_INPUT : decoded;
+					ZEND_ASSERT(decoded >= 0x10000); /* Not an overlong code unit */
+					*out++ = decoded;
 				}
 			} else {
 				*out++ = MBFL_BAD_INPUT;