diff --git a/CMakeLists.txt b/CMakeLists.txt
index 65f97ca..da60b6c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -80,3 +80,4 @@ set_property(TEST invalid_not_bridge.frank PROPERTY WILL_FAIL true)
 set_property(TEST region_bad_1.frank PROPERTY WILL_FAIL true)
 set_property(TEST region_bad_2.frank PROPERTY WILL_FAIL true)
 set_property(TEST fail_cross_region_ref.frank PROPERTY WILL_FAIL true)
+set_property(TEST region_close_fail_1.frank PROPERTY WILL_FAIL true)
diff --git a/docs/builtin.md b/docs/builtin.md
index 92bc90e..ab7830e 100644
--- a/docs/builtin.md
+++ b/docs/builtin.md
@@ -44,7 +44,7 @@ Forces the given region to close by invalidating all local references into to re
 
 This function will also correct all dirty LRCs.
 
-#### `try_close(reg)`
+#### `is_closed(reg)`
 
 Checks if the given region can be closed. If the LRC is dirty or subreagions are open, it will correct all dirty LRCs.
 
diff --git a/src/rt/core/builtin.cc b/src/rt/core/builtin.cc
index 55cac70..edb3670 100644
--- a/src/rt/core/builtin.cc
+++ b/src/rt/core/builtin.cc
@@ -300,7 +300,7 @@ namespace rt::core
       close_function_impl(frame, args, true);
       return std::nullopt;
     });
-    add_builtin("try_close", [](auto frame, auto args) {
+    add_builtin("is_closed", [](auto frame, auto args) {
       auto result = close_function_impl(frame, args, false);
       auto result_obj = rt::get_bool(result);
       // The return will be linked to the frame by the interpreter, but the RC
diff --git a/src/rt/objects/region.cc b/src/rt/objects/region.cc
index 574e0e7..fdf84ed 100644
--- a/src/rt/objects/region.cc
+++ b/src/rt/objects/region.cc
@@ -308,6 +308,11 @@ namespace rt::objects
          Region::is_ancestor(dst_reg, to_close_reg));
       if (invalidate)
       {
+        if (e.key == PrototypeField)
+        {
+          ui::error("Can't close the region due to this prototype", e);
+        }
+
         auto old = src->set(e.key, nullptr);
         assert(old == dst);
         add_reference(src, nullptr);
diff --git a/tests/regions/region_close_1.frank b/tests/regions/region_close_1.frank
index 9f1ad80..abae05c 100644
--- a/tests/regions/region_close_1.frank
+++ b/tests/regions/region_close_1.frank
@@ -14,8 +14,8 @@ lref2 = r2.b.c
 
 # r2.LRC is now 3, from: r2, lref1, lref2
 
-# Fail `try_close()`
-if try_close(r2):
+# Fail `is_closed()`
+if is_closed(r2):
     unreachable()
 
 # Force close
diff --git a/tests/regions/region_close_2.frank b/tests/regions/region_close_2.frank
index eefc9d1..86a85b3 100644
--- a/tests/regions/region_close_2.frank
+++ b/tests/regions/region_close_2.frank
@@ -9,8 +9,8 @@ r1.r2.b = {}
 # r2.LRC is now 0
 # r2.LRC will be 1, from the value on the stack
 
-# Succeed `try_close()`
-if try_close(r1.r2):
+# Succeed `is_closed()`
+if is_closed(r1.r2):
     pass()
 else:
     unreachable()
diff --git a/tests/regions/region_close_3.frank b/tests/regions/region_close_3.frank
index 7ba5348..a1831d9 100644
--- a/tests/regions/region_close_3.frank
+++ b/tests/regions/region_close_3.frank
@@ -21,8 +21,8 @@ lref2 = r2.b.c
 freeze(r2.b)
 r2 = None
 
-# Succeed `try_close()` after LRC cleaning
-if try_close(r1.r2):
+# Succeed `is_closed()` after LRC cleaning
+if is_closed(r1.r2):
     pass()
 else:
     unreachable()
diff --git a/tests/regions/region_close_4.frank b/tests/regions/region_close_4.frank
index 564709a..966e9c2 100644
--- a/tests/regions/region_close_4.frank
+++ b/tests/regions/region_close_4.frank
@@ -9,7 +9,7 @@ r0.r1.r2.r3.a = {}
 lref = r0.r1.r2.r3.a
 
 # Fail try close, since r3 is open
-if try_close(r0.r1):
+if is_closed(r0.r1):
     unreachable()
 
 # Force close
diff --git a/tests/regions/region_close_5.frank b/tests/regions/region_close_5.frank
index 4559ff1..76fba9f 100644
--- a/tests/regions/region_close_5.frank
+++ b/tests/regions/region_close_5.frank
@@ -1,9 +1,9 @@
 # Helper to close the region while a ref is on the stack
 def close_and_get(r1):
     # Check that the region is currently open
-    if try_close(r1.r2):
+    if is_closed(r1.r2):
         unreachable()
-    
+
     # Force close it
     close(r1.r2)
 
diff --git a/tests/regions/region_close_fail_1.frank b/tests/regions/region_close_fail_1.frank
new file mode 100644
index 0000000..c316afc
--- /dev/null
+++ b/tests/regions/region_close_fail_1.frank
@@ -0,0 +1,11 @@
+# Construct regions
+r1 = Region()
+
+# Create a local string to share the `String` prototype
+local_string = "Hello Local"
+
+# This also moves the `String` prototype
+r1.something = "Important string"
+
+# This should fail, since implicit freezing is disabled
+close(r1)