diff --git a/src/core/compile.c b/src/core/compile.c
index fc668d4e..c11ec6da 100644
--- a/src/core/compile.c
+++ b/src/core/compile.c
@@ -414,6 +414,11 @@ found:
         ret.envindex = -1;
         return ret;
     }
+    if (ret.index > 0xFF) {
+        janetc_error(c, janet_formatc("too many closure upvalues: %S", sym));
+        ret.envindex = -1;
+        return ret;
+    }
 
     /* non-local scope needs to expose its environment */
     JanetScope *original_scope = scope;
diff --git a/src/core/emit.c b/src/core/emit.c
index 32e20fc9..e4e76d49 100644
--- a/src/core/emit.c
+++ b/src/core/emit.c
@@ -125,6 +125,10 @@ static void janetc_movenear(JanetCompiler *c,
                         JOP_GET_INDEX);
         }
     } else if (src.envindex >= 0) {
+        /* Convert src to near reg */
+        if (src.index > 255) {
+            janet_assert(0, "upvalue index > 255, cannot load");
+        }
         janetc_emit(c,
                     ((uint32_t)(src.index) << 24) |
                     ((uint32_t)(src.envindex) << 16) |
@@ -158,6 +162,9 @@ static void janetc_moveback(JanetCompiler *c,
             janetc_emit(c, JOP_MOVE_NEAR | ((uint32_t)(src) << 16) | ((uint32_t) newsrc << 8));
             src = newsrc;
         }
+        if (dest.index > 255) {
+            janet_assert(0, "upvalue index > 255, cannot store");
+        }
         janetc_emit(c,
                     ((uint32_t)(dest.index) << 24) |
                     ((uint32_t)(dest.envindex) << 16) |
diff --git a/src/core/specials.c b/src/core/specials.c
index 8798b7fe..25da14f3 100644
--- a/src/core/specials.c
+++ b/src/core/specials.c
@@ -963,6 +963,10 @@ static JanetSlot janetc_while(JanetFopts opts, int32_t argn, const Janet *argv)
         JanetFuncDef *def = janetc_pop_funcdef(c, janet_cstring("while"));
         janet_def_addflags(def);
         int32_t defindex = janetc_addfuncdef(c, def);
+        if (defindex > 255) {
+            janetc_error(c, janet_cstring("too many closures"));
+            return janetc_cslot(janet_wrap_nil());
+        }
         /* Do basic optimization */
         if (!(c->scope->flags & JANET_SCOPE_UNUSED)) {
             janet_bytecode_optimize(def, c->optimize);
@@ -1212,6 +1216,10 @@ static JanetSlot janetc_fn(JanetFopts opts, int32_t argn, const Janet *argv) {
 
     janet_def_addflags(def);
     defindex = janetc_addfuncdef(c, def);
+    if (defindex > 255) {
+        errmsg = "too many closures";
+        goto error;
+    }
 
     /* Ensure enough slots for vararg function. */
     if (arity + vararg > def->slotcount) def->slotcount = arity + vararg;