diff --git a/src/asm/state.rs b/src/asm/state.rs
index 42eecf7b..6afb287d 100644
--- a/src/asm/state.rs
+++ b/src/asm/state.rs
@@ -270,6 +270,15 @@ impl State
 		report: diagn::RcReport)
 		-> Result<(), ()>
 	{
+		if self.banks.len() > 0 && self.bankdata[0].cur_bit_offset != 0
+		{
+			report.error_span(
+				"cannot create new bank if the default bank has already been used",
+				&bank.decl_span.as_ref().unwrap());
+
+			return Err(());
+		}
+
 		if bank.output_offset.is_some()
 		{
 			for j in 1..self.banks.len()
diff --git a/tests/bank_default.asm b/tests/bank_default.asm
new file mode 100644
index 00000000..7e5021f7
--- /dev/null
+++ b/tests/bank_default.asm
@@ -0,0 +1,20 @@
+; :::
+
+#d8 1, 2, 3, 4 ; = 0x01020304
+
+; :::
+
+#d8 1, 2, 3, 4
+#bankdef a_new_bank {} ; error: default bank
+
+; :::
+
+#res 4
+#bankdef a_new_bank {} ; error: default bank
+
+; :::
+
+x = 0x25
+label:
+#bankdef a_new_bank {}
+; = 0x
\ No newline at end of file
diff --git a/web/customasm.gc.wasm b/web/customasm.gc.wasm
index 654e5a96..59306336 100644
Binary files a/web/customasm.gc.wasm and b/web/customasm.gc.wasm differ