diff --git a/bin/sim.m b/bin/sim.m
index 73ecde23..2e6c7470 100644
--- a/bin/sim.m
+++ b/bin/sim.m
@@ -214,18 +214,20 @@ rl_set_prompt(const char *prompt)
 static void
 refresh_repl_prompt(NSString *top_level, bool clear)
 {
-    static int previous_prompt_length = 0;
-    rl_set_prompt([current_repl_prompt(top_level) UTF8String]);
-    if (clear) {
-	putchar('\r');
-	for (int i = 0; i < previous_prompt_length; i++) {
-	    putchar(' ');
-	}
-	putchar('\r');
-	//printf("\n\033[F\033[J"); // Clear.
-	rl_forced_update_display();
-    }
-    previous_prompt_length = strlen(rl_prompt);
+    dispatch_async(dispatch_get_main_queue(), ^{
+        static int previous_prompt_length = 0;
+        rl_set_prompt([current_repl_prompt(top_level) UTF8String]);
+        if (clear) {
+    	putchar('\r');
+    	for (int i = 0; i < previous_prompt_length; i++) {
+    	    putchar(' ');
+    	}
+    	putchar('\r');
+    	//printf("\n\033[F\033[J"); // Clear.
+    	rl_forced_update_display();
+        }
+        previous_prompt_length = strlen(rl_prompt);
+    });
 }
 
 static void