diff --git a/src/fcgi.c b/src/fcgi.c
index 1d1a82b4c..0ff76156f 100644
--- a/src/fcgi.c
+++ b/src/fcgi.c
@@ -198,7 +198,7 @@ size_t fcgi_decode_param(const struct buffer *in, size_t o, struct fcgi_param *p
 		len += 4;
 	}
 
-	if (data < nlen + vlen)
+	if (data < o + nlen + vlen)
 		return 0;
 
 	p->n = ist2(b_peek(in, o), nlen);
@@ -253,7 +253,7 @@ size_t fcgi_aligned_decode_param(const struct buffer *in, size_t o, struct fcgi_
 		len += 4;
 	}
 
-	if (data < nlen + vlen)
+	if (data < o + nlen + vlen)
 		return 0;
 
 	p->n = ist2(in->area + o, nlen);