diff --git a/tests/parser/features/test_init.py b/tests/parser/features/test_init.py
index 3ef7a440a5..feeabe311a 100644
--- a/tests/parser/features/test_init.py
+++ b/tests/parser/features/test_init.py
@@ -15,12 +15,13 @@ def __init__(a: uint256):
     assert c.val() == 123
 
     # Make sure the init code does not access calldata
-    opcodes = vyper.compile_code(code, ["opcodes"])["opcodes"].split(" ")
-    ir_return_idx = opcodes.index("JUMP")
+    assembly = vyper.compile_code(code, ["asm"])["asm"].split(" ")
+    ir_return_idx_start = assembly.index("{")
+    ir_return_idx_end = assembly.index("}")
 
-    assert "CALLDATALOAD" in opcodes
-    assert "CALLDATACOPY" not in opcodes[:ir_return_idx]
-    assert "CALLDATALOAD" not in opcodes[:ir_return_idx]
+    assert "CALLDATALOAD" in assembly
+    assert "CALLDATACOPY" not in assembly[:ir_return_idx_start] + assembly[ir_return_idx_end:]
+    assert "CALLDATALOAD" not in assembly[:ir_return_idx_start] + assembly[ir_return_idx_end:]
 
 
 def test_init_calls_internal(get_contract, assert_compile_failed, assert_tx_failed):
diff --git a/vyper/codegen/function_definitions/external_function.py b/vyper/codegen/function_definitions/external_function.py
index 202c75a448..6de5bab16f 100644
--- a/vyper/codegen/function_definitions/external_function.py
+++ b/vyper/codegen/function_definitions/external_function.py
@@ -123,7 +123,24 @@ def handler_for(calldata_kwargs, default_kwargs):
 
         ret.append(["goto", sig.external_function_base_entry_label])
 
-        ret = ["if", ["eq", "_calldata_method_id", method_id], ret]
+        method_id_check = ["eq", "_calldata_method_id", method_id]
+
+        # if there is a function whose selector is 0, it won't be distinguished
+        # from the case where nil calldata is supplied, b/c calldataload loads
+        # 0s past the end of physical calldata (cf. yellow paper).
+        # since supplying 0 calldata is expected to trigger the fallback fn,
+        # we check that calldatasize > 0, which distinguishes the 0 selector
+        # from the fallback function "selector"
+        # (equiv. to "all selectors not in the selector table").
+
+        # note: cases where not enough calldata is supplied (besides
+        # calldatasize==0) are not addressed here b/c a calldatasize
+        # well-formedness check is already present in the function body
+        # as part of abi validation
+        if method_id.value == 0:
+            method_id_check = ["and", ["gt", "calldatasize", 0], method_id_check]
+
+        ret = ["if", method_id_check, ret]
         return ret
 
     ret = ["seq"]
diff --git a/vyper/codegen/module.py b/vyper/codegen/module.py
index fdca675819..71f9ed552d 100644
--- a/vyper/codegen/module.py
+++ b/vyper/codegen/module.py
@@ -120,9 +120,6 @@ def _runtime_ir(runtime_functions, all_sigs, global_ctx):
 
     runtime = [
         "seq",
-        # check that calldatasize is at least 4, otherwise
-        # calldataload will load zeros (cf. yellow paper).
-        ["if", ["lt", "calldatasize", 4], ["goto", "fallback"]],
         ["with", "_calldata_method_id", shr(224, ["calldataload", 0]), selector_section],
         close_selector_section,
         ["label", "fallback", ["var_list"], fallback_ir],