diff --git a/src/mod_cgi.c b/src/mod_cgi.c
index c8c0d01c..c19ffed6 100644
--- a/src/mod_cgi.c
+++ b/src/mod_cgi.c
@@ -500,7 +500,7 @@ static int cgi_env_add(void *venv, const char *key, size_t key_len, const char *
 	env_accum *env = venv;
 	char *dst;
 
-	if (!key || !val) return -1;
+	if (!key || (!val && val_len)) return -1;
 
 	if (env->size - env->used < key_len + val_len + 2) {
 		if (0 == env->size) env->size = 4096;
diff --git a/src/mod_fastcgi.c b/src/mod_fastcgi.c
index 3c3f992c..90f9b622 100644
--- a/src/mod_fastcgi.c
+++ b/src/mod_fastcgi.c
@@ -149,7 +149,7 @@ static int fcgi_env_add(void *venv, const char *key, size_t key_len, const char
 	size_t len_enc_len = 0;
 	char *dst;
 
-	if (!key || !val) return -1;
+	if (!key || (!val && val_len)) return -1;
 
 	len = key_len + val_len;
 
diff --git a/src/mod_scgi.c b/src/mod_scgi.c
index 857cb3ac..636662d1 100644
--- a/src/mod_scgi.c
+++ b/src/mod_scgi.c
@@ -149,7 +149,7 @@ static int scgi_env_add_scgi(void *venv, const char *key, size_t key_len, const
 	char *dst;
 	size_t len;
 
-	if (!key || !val) return -1;
+	if (!key || (!val && val_len)) return -1;
 
 	len = key_len + val_len + 2;