diff --git a/cmd-set-environment.c b/cmd-set-environment.c
index 864e1d9b..2c0010af 100644
--- a/cmd-set-environment.c
+++ b/cmd-set-environment.c
@@ -79,13 +79,13 @@ cmd_set_environment_exec(struct cmd *self, struct cmd_q *cmdq)
 			cmdq_error(cmdq, "can't specify a value with -r");
 			return (CMD_RETURN_ERROR);
 		}
-		environ_set(env, name, NULL);
+		environ_clear(env, name);
 	} else {
 		if (value == NULL) {
 			cmdq_error(cmdq, "no value specified");
 			return (CMD_RETURN_ERROR);
 		}
-		environ_set(env, name, value);
+		environ_set(env, name, "%s", value);
 	}
 
 	return (CMD_RETURN_NORMAL);
diff --git a/environ.c b/environ.c
index 43a9ce01..de560896 100644
--- a/environ.c
+++ b/environ.c
@@ -83,8 +83,12 @@ environ_copy(struct environ *srcenv, struct environ *dstenv)
 {
 	struct environ_entry	*envent;
 
-	RB_FOREACH(envent, environ, srcenv)
-		environ_set(dstenv, envent->name, envent->value);
+	RB_FOREACH(envent, environ, srcenv) {
+		if (envent->value == NULL)
+			environ_clear(dstenv, envent->name);
+		else
+			environ_set(dstenv, envent->name, "%s", envent->value);
+	}
 }
 
 /* Find an environment variable. */
@@ -99,23 +103,37 @@ environ_find(struct environ *env, const char *name)
 
 /* Set an environment variable. */
 void
-environ_set(struct environ *env, const char *name, const char *value)
+environ_set(struct environ *env, const char *name, const char *fmt, ...)
+{
+	struct environ_entry	*envent;
+	va_list			 ap;
+
+	va_start(ap, fmt);
+	if ((envent = environ_find(env, name)) != NULL) {
+		free(envent->value);
+		xvasprintf(&envent->value, fmt, ap);
+	} else {
+		envent = xmalloc(sizeof *envent);
+		envent->name = xstrdup(name);
+		xvasprintf(&envent->value, fmt, ap);
+		RB_INSERT(environ, env, envent);
+	}
+	va_end(ap);
+}
+
+/* Clear an environment variable. */
+void
+environ_clear(struct environ *env, const char *name)
 {
 	struct environ_entry	*envent;
 
 	if ((envent = environ_find(env, name)) != NULL) {
 		free(envent->value);
-		if (value != NULL)
-			envent->value = xstrdup(value);
-		else
-			envent->value = NULL;
+		envent->value = NULL;
 	} else {
 		envent = xmalloc(sizeof *envent);
 		envent->name = xstrdup(name);
-		if (value != NULL)
-			envent->value = xstrdup(value);
-		else
-			envent->value = NULL;
+		envent->value = NULL;
 		RB_INSERT(environ, env, envent);
 	}
 }
@@ -134,7 +152,7 @@ environ_put(struct environ *env, const char *var)
 	name = xstrdup(var);
 	name[strcspn(name, "=")] = '\0';
 
-	environ_set(env, name, value);
+	environ_set(env, name, "%s", value);
 	free(name);
 }
 
@@ -166,9 +184,9 @@ environ_update(const char *vars, struct environ *srcenv,
 	copyvars = next = xstrdup(vars);
 	while ((var = strsep(&next, " ")) != NULL) {
 		if ((envent = environ_find(srcenv, var)) == NULL)
-			environ_set(dstenv, var, NULL);
+			environ_clear(dstenv, var);
 		else
-			environ_set(dstenv, envent->name, envent->value);
+			environ_set(dstenv, envent->name, "%s", envent->value);
 	}
 	free(copyvars);
 }
diff --git a/server-fn.c b/server-fn.c
index 3e4b6116..a22c964d 100644
--- a/server-fn.c
+++ b/server-fn.c
@@ -34,20 +34,19 @@ void		server_callback_identify(int, short, void *);
 void
 server_fill_environ(struct session *s, struct environ *env)
 {
-	char	var[PATH_MAX], *term;
-	u_int	idx;
-	long	pid;
+	char	*term;
+	u_int	 idx;
+	long	 pid;
 
 	if (s != NULL) {
 		term = options_get_string(global_options, "default-terminal");
-		environ_set(env, "TERM", term);
+		environ_set(env, "TERM", "%s", term);
 
 		idx = s->id;
 	} else
 		idx = (u_int)-1;
 	pid = getpid();
-	xsnprintf(var, sizeof var, "%s,%ld,%u", socket_path, pid, idx);
-	environ_set(env, "TMUX", var);
+	environ_set(env, "TMUX", "%s,%ld,%u", socket_path, pid, idx);
 }
 
 void
diff --git a/tmux.1 b/tmux.1
index 8fa6ad55..454af1d2 100644
--- a/tmux.1
+++ b/tmux.1
@@ -750,7 +750,7 @@ If
 is given, all sessions but the specified one is killed.
 The
 .Fl C
-clears alerts (bell, activity, or silence) in all windows linked to the
+flag clears alerts (bell, activity, or silence) in all windows linked to the
 session.
 .It Xo Ic list-clients
 .Op Fl F Ar format
diff --git a/tmux.c b/tmux.c
index 7d49dd30..68f0092d 100644
--- a/tmux.c
+++ b/tmux.c
@@ -273,7 +273,7 @@ main(int argc, char **argv)
 	for (var = environ; *var != NULL; var++)
 		environ_put(global_environ, *var);
 	if (getcwd(tmp, sizeof tmp) != NULL)
-		environ_set(global_environ, "PWD", tmp);
+		environ_set(global_environ, "PWD", "%s", tmp);
 
 	global_options = options_create(NULL);
 	options_table_populate_tree(OPTIONS_TABLE_SERVER, global_options);
diff --git a/tmux.h b/tmux.h
index f4638bc8..2acfc75f 100644
--- a/tmux.h
+++ b/tmux.h
@@ -1567,7 +1567,9 @@ struct environ_entry *environ_first(struct environ *);
 struct environ_entry *environ_next(struct environ_entry *);
 void	environ_copy(struct environ *, struct environ *);
 struct environ_entry *environ_find(struct environ *, const char *);
-void	environ_set(struct environ *, const char *, const char *);
+void printflike(3, 4) environ_set(struct environ *, const char *, const char *,
+	    ...);
+void	environ_clear(struct environ *, const char *);
 void	environ_put(struct environ *, const char *);
 void	environ_unset(struct environ *, const char *);
 void	environ_update(const char *, struct environ *, struct environ *);
@@ -1739,7 +1741,7 @@ RB_PROTOTYPE(key_tables, key_table, entry, key_table_cmp);
 extern struct key_tables key_tables;
 int	 key_table_cmp(struct key_table *, struct key_table *);
 int	 key_bindings_cmp(struct key_binding *, struct key_binding *);
-struct	 key_table *key_bindings_get_table(const char *, int);
+struct key_table *key_bindings_get_table(const char *, int);
 void	 key_bindings_unref_table(struct key_table *);
 void	 key_bindings_add(const char *, key_code, int, struct cmd_list *);
 void	 key_bindings_remove(const char *, key_code);
diff --git a/window.c b/window.c
index 8ee517a1..deda2895 100644
--- a/window.c
+++ b/window.c
@@ -808,7 +808,7 @@ window_pane_spawn(struct window_pane *wp, int argc, char **argv,
     struct termios *tio, char **cause)
 {
 	struct winsize	 ws;
-	char		*argv0, *cmd, **argvp, paneid[16];
+	char		*argv0, *cmd, **argvp;
 	const char	*ptr, *first, *home;
 	struct termios	 tio2;
 	int		 i;
@@ -863,9 +863,8 @@ window_pane_spawn(struct window_pane *wp, int argc, char **argv,
 		closefrom(STDERR_FILENO + 1);
 
 		if (path != NULL)
-			environ_set(env, "PATH", path);
-		xsnprintf(paneid, sizeof paneid, "%%%u", wp->id);
-		environ_set(env, "TMUX_PANE", paneid);
+			environ_set(env, "PATH", "%s", path);
+		environ_set(env, "TMUX_PANE", "%%%u", wp->id);
 		environ_push(env);
 
 		clear_signals(1);