diff --git a/frontend/dockerfile/parser/directives.go b/frontend/dockerfile/parser/directives.go
index c29d18b8f11d..4f1e1300ec4e 100644
--- a/frontend/dockerfile/parser/directives.go
+++ b/frontend/dockerfile/parser/directives.go
@@ -112,10 +112,14 @@ func (d *DirectiveParser) ParseAll(data []byte) ([]*Directive, error) {
 // This allows for a flexible range of input formats, and appropriate syntax
 // selection.
 func DetectSyntax(dt []byte) (string, string, []Range, bool) {
-	return ParseDirective(keySyntax, dt)
+	return parseDirective(keySyntax, dt, true)
 }
 
 func ParseDirective(key string, dt []byte) (string, string, []Range, bool) {
+	return parseDirective(key, dt, false)
+}
+
+func parseDirective(key string, dt []byte, anyFormat bool) (string, string, []Range, bool) {
 	dt = discardBOM(dt)
 	dt, hadShebang, err := discardShebang(dt)
 	if err != nil {
@@ -132,6 +136,10 @@ func ParseDirective(key string, dt []byte) (string, string, []Range, bool) {
 		return syntax, cmdline, loc, true
 	}
 
+	if !anyFormat {
+		return "", "", nil, false
+	}
+
 	// use directive with different comment prefix, and search for //key=
 	directiveParser = DirectiveParser{line: line}
 	directiveParser.setComment("//")
diff --git a/frontend/dockerfile/parser/directives_test.go b/frontend/dockerfile/parser/directives_test.go
index 651120b29118..dd633dddc2fb 100644
--- a/frontend/dockerfile/parser/directives_test.go
+++ b/frontend/dockerfile/parser/directives_test.go
@@ -158,27 +158,6 @@ RUN ls
 
 	dt = `//check=skip=all
 //key=value`
-	ref, _, _, ok = ParseDirective("check", []byte(dt))
-	require.True(t, ok)
-	require.Equal(t, "skip=all", ref)
-
-	dt = `#!/bin/sh
-//check=skip=all`
-	ref, _, _, ok = ParseDirective("check", []byte(dt))
-	require.True(t, ok)
-	require.Equal(t, "skip=all", ref)
-
-	dt = `{"check": "skip=all"}`
-	ref, _, _, ok = ParseDirective("check", []byte(dt))
-	require.True(t, ok)
-	require.Equal(t, "skip=all", ref)
-
-	dt = `{"check": "foo"`
-	_, _, _, ok = ParseDirective("check", []byte(dt))
-	require.False(t, ok)
-
-	dt = `{"check": "foo"}
-# syntax=bar`
 	_, _, _, ok = ParseDirective("check", []byte(dt))
 	require.False(t, ok)
 }