diff --git a/environ.c b/environ.c
index 7709f29d..5d06afbf 100644
--- a/environ.c
+++ b/environ.c
@@ -206,10 +206,14 @@ environ_push(struct environ *env)
 
 /* Log the environment. */
 void
-environ_log(struct environ *env)
+environ_log(struct environ *env, const char *prefix)
 {
 	struct environ_entry	*envent;
 
-	RB_FOREACH(envent, environ, env)
-	    log_debug("%s=%s", envent->name, envent->value);
+	RB_FOREACH(envent, environ, env) {
+		if (envent->value != NULL && *envent->name != '\0') {
+			log_debug("%s%s=%s", prefix, envent->name,
+			    envent->value);
+		}
+	}
 }
diff --git a/tmux.h b/tmux.h
index 95d942f2..bad1d87e 100644
--- a/tmux.h
+++ b/tmux.h
@@ -1693,7 +1693,7 @@ void	environ_put(struct environ *, const char *);
 void	environ_unset(struct environ *, const char *);
 void	environ_update(const char *, struct environ *, struct environ *);
 void	environ_push(struct environ *);
-void	environ_log(struct environ *);
+void	environ_log(struct environ *, const char *);
 
 /* tty.c */
 void	tty_create_log(void);
diff --git a/window.c b/window.c
index 2f158e3a..c58313d5 100644
--- a/window.c
+++ b/window.c
@@ -842,7 +842,7 @@ window_pane_spawn(struct window_pane *wp, int argc, char **argv,
 	log_debug("spawn: %s -- %s", wp->shell, cmd);
 	for (i = 0; i < wp->argc; i++)
 		log_debug("spawn: argv[%d] = %s", i, wp->argv[i]);
-	environ_log(env);
+	environ_log(env, "spawn: ");
 
 	memset(&ws, 0, sizeof ws);
 	ws.ws_col = screen_size_x(&wp->base);