diff --git a/Zend/zend_compile.c b/Zend/zend_compile.c
index 38d378a4175bb..52b3417234cf7 100644
--- a/Zend/zend_compile.c
+++ b/Zend/zend_compile.c
@@ -1997,7 +1997,7 @@ ZEND_API size_t zend_dirname(char *path, size_t len)
 	}
 
 	/* Strip trailing slashes */
-	while (end >= path && IS_SLASH_P(end)) {
+	while (end >= path && IS_SLASH_P_EX(end, end == path)) {
 		end--;
 	}
 	if (end < path) {
@@ -2008,7 +2008,7 @@ ZEND_API size_t zend_dirname(char *path, size_t len)
 	}
 
 	/* Strip filename */
-	while (end >= path && !IS_SLASH_P(end)) {
+	while (end >= path && !IS_SLASH_P_EX(end, end == path)) {
 		end--;
 	}
 	if (end < path) {
@@ -2019,7 +2019,7 @@ ZEND_API size_t zend_dirname(char *path, size_t len)
 	}
 
 	/* Strip slashes which came before the file name */
-	while (end >= path && IS_SLASH_P(end)) {
+	while (end >= path && IS_SLASH_P_EX(end, end == path)) {
 		end--;
 	}
 	if (end < path) {
diff --git a/Zend/zend_virtual_cwd.h b/Zend/zend_virtual_cwd.h
index 728e3ba69d888..28cbf6300b8ec 100644
--- a/Zend/zend_virtual_cwd.h
+++ b/Zend/zend_virtual_cwd.h
@@ -73,8 +73,11 @@ typedef unsigned short mode_t;
 #define DEFAULT_SLASH '\\'
 #define DEFAULT_DIR_SEPARATOR	';'
 #define IS_SLASH(c)	((c) == '/' || (c) == '\\')
+// IS_SLASH_P() may read the previous char on Windows, which may be OOB; use IS_SLASH_P_EX() instead
 #define IS_SLASH_P(c)	(*(c) == '/' || \
         (*(c) == '\\' && !IsDBCSLeadByte(*(c-1))))
+#define IS_SLASH_P_EX(c, first_byte)	(*(c) == '/' || \
+        (*(c) == '\\' && ((first_byte) || !IsDBCSLeadByte(*(c-1)))))
 
 /* COPY_WHEN_ABSOLUTE is 2 under Win32 because by chance both regular absolute paths
    in the file system and UNC paths need copying of two characters */
@@ -98,7 +101,9 @@ typedef unsigned short mode_t;
 #endif
 
 #define IS_SLASH(c)	((c) == '/')
+// IS_SLASH_P() may read the previous char on Windows, which may be OOB; use IS_SLASH_P_EX() instead
 #define IS_SLASH_P(c)	(*(c) == '/')
+#define IS_SLASH_P_EX(c, first_byte) IS_SLASH_P(c)
 
 #endif