diff --git a/modules/caddyhttp/headers/headers.go b/modules/caddyhttp/headers/headers.go
index d5237236..f8d3fdcc 100644
--- a/modules/caddyhttp/headers/headers.go
+++ b/modules/caddyhttp/headers/headers.go
@@ -332,7 +332,10 @@ func (rww *responseWriterWrapper) WriteHeader(status int) {
 	if rww.wroteHeader {
 		return
 	}
-	rww.wroteHeader = true
+	// 1xx responses aren't final; just informational
+	if status < 100 || status > 199 {
+		rww.wroteHeader = true
+	}
 	if rww.require == nil || rww.require.Match(status, rww.ResponseWriterWrapper.Header()) {
 		if rww.headerOps != nil {
 			rww.headerOps.ApplyTo(rww.ResponseWriterWrapper.Header(), rww.replacer)