diff --git a/src/engine/Runtime.v3 b/src/engine/Runtime.v3
index 623fa2526..c37e92193 100644
--- a/src/engine/Runtime.v3
+++ b/src/engine/Runtime.v3
@@ -378,15 +378,7 @@ component Runtime {
 	def cast(instance: Instance, nullable: bool, ht_val: int, val: Value) -> bool {
 		match (val) {
 			Ref(obj) => return if(obj == null, nullable, castObject(instance, ht_val, obj));
-			I31(val) => {
-				match (ht_val) {
-					BpHeapTypeCode.EXTERN.val,
-					BpHeapTypeCode.ANY.val,
-					BpHeapTypeCode.EQ.val,
-					BpHeapTypeCode.I31.val => return true;
-					_ => return false;
-				}
-			}
+			I31(val) => return castI31(instance, ht_val);
 			_ => return false;
 		}
 	}
@@ -399,7 +391,7 @@ component Runtime {
 			_ => return false;
 		}
 	}
-	// caller do nullable
+	// Caller must handle nullability.
 	def castObject(instance: Instance, ht_val: int, obj: Object) -> bool {
 		match (ht_val) {
 			BpHeapTypeCode.FUNC.val => return Function.?(obj);
diff --git a/src/engine/v3/V3Interpreter.v3 b/src/engine/v3/V3Interpreter.v3
index dcbb4c9b1..95c72b94f 100644
--- a/src/engine/v3/V3Interpreter.v3
+++ b/src/engine/v3/V3Interpreter.v3
@@ -892,7 +892,7 @@ class V3Interpreter extends WasmStack {
 			REF_TEST_NULL => {
 				var nullable = (opcode == Opcode.REF_TEST_NULL);
 				var ht_val = codeptr.read_sleb32();
-				var result = doRuntimeCast(frame.func.instance, nullable, ht_val, pop());
+				var result = Runtime.cast(frame.func.instance, nullable, ht_val, pop());
 				pushz(result);
 			}
 			REF_CAST,
@@ -900,14 +900,14 @@ class V3Interpreter extends WasmStack {
 				var nullable = (opcode == Opcode.REF_CAST_NULL);
 				var ht_val = codeptr.read_sleb32();
 				var val = pop();
-				var result = doRuntimeCast(frame.func.instance, nullable, ht_val, val);
+				var result = Runtime.cast(frame.func.instance, nullable, ht_val, val);
 				if (!result) trap(TrapReason.FAILED_CAST);
 				else push(val);
 			}
 			BR_ON_CAST => {
 				var imm = codeptr.read_BrOnCastImm();
 				var val = pop();
-				var result = doRuntimeCast(frame.func.instance, imm.null2(), imm.ht2, val);
+				var result = Runtime.cast(frame.func.instance, imm.null2(), imm.ht2, val);
 				push(val);
 				if (result) codeptr.at(doGoto(pc));
 				else doFallthru();
@@ -915,7 +915,7 @@ class V3Interpreter extends WasmStack {
 			BR_ON_CAST_FAIL => {
 				var imm = codeptr.read_BrOnCastImm();
 				var val = pop();
-				var result = doRuntimeCast(frame.func.instance, imm.null2(), imm.ht2, val);
+				var result = Runtime.cast(frame.func.instance, imm.null2(), imm.ht2, val);
 				push(val);
 				if (!result) codeptr.at(doGoto(pc));
 				else doFallthru();
@@ -2050,13 +2050,6 @@ class V3Interpreter extends WasmStack {
 			last_fp = f.fp;
 		}
 	}
-	private def doRuntimeCast(instance: Instance, nullable: bool, ht_val: int, val: Value) -> bool {
-		match (val) {
-			Ref(obj) => return if(obj == null, nullable, Runtime.castObject(instance, ht_val, obj));
-			I31 => return Runtime.castI31(instance, ht_val);
-			_ => return false;
-		}
-	}
 }
 
 class V3Frame {
diff --git a/src/engine/x86-64/X86_64Runtime.v3 b/src/engine/x86-64/X86_64Runtime.v3
index 607bc5254..965034801 100644
--- a/src/engine/x86-64/X86_64Runtime.v3
+++ b/src/engine/x86-64/X86_64Runtime.v3
@@ -213,10 +213,11 @@ component X86_64Runtime {
 		curStack.pushRspPointer(Pointer.NULL);
 		return null;
 	}
-	def runtime_doCast(stack: X86_64Stack, instance: Instance, nullable: byte, ht_val: int) -> bool {
+	def runtime_doCast(stack: X86_64Stack, instance: Instance, nullable: byte, ht_val: int) -> bool { // XXX: inline into caller
 		var isI31 = stack.topObjectIsI31();
-		if (isI31) return Runtime.castI31(instance, ht_val);
-		else {
+		if (isI31) {
+			return Runtime.castI31(instance, ht_val);
+		} else {
 			var obj = stack.peekObject();
 			return if(obj == null, (nullable & 2) != 0, Runtime.castObject(instance, ht_val, obj));
 		}
diff --git a/src/monitors/ControlInstrumenter.v3 b/src/monitors/ControlInstrumenter.v3
index 129834388..c2c857a7e 100644
--- a/src/monitors/ControlInstrumenter.v3
+++ b/src/monitors/ControlInstrumenter.v3
@@ -136,12 +136,7 @@ private class CiBrOnCastProbe(nullable: bool, ht_val: int, success_taken: bool)
 	def fire(loc: DynamicLoc) -> ProbeAction {
 		var accessor = loc.frame.getFrameAccessor();
 		var instance = accessor.func().instance;
-		var result: bool;
-		match (accessor.getOperand(0)) {
-			Ref(obj) => result = if(obj == null, nullable, Runtime.castObject(instance, ht_val, obj));
-			I31 => result = Runtime.castI31(instance, ht_val);
-			_ => result = false;
-		}
+		var result = Runtime.cast(instance, nullable, ht_val, accessor.getOperand(0));
 		taken[if(result == success_taken, 0, 1)]++;
 		return ProbeAction.Continue;
 	}
diff --git a/src/monitors/LoopTraceMonitor.v3 b/src/monitors/LoopTraceMonitor.v3
index 43eaaf49e..39f2c1159 100644
--- a/src/monitors/LoopTraceMonitor.v3
+++ b/src/monitors/LoopTraceMonitor.v3
@@ -239,12 +239,7 @@ private class TraceBrOnCastProbe(trace: TraceBuffer, nullable: bool, ht_val: int
 		// XXX: introduce a utility for taken/not taken branches
 		var accessor = loc.frame.getFrameAccessor();
 		var instance = accessor.func().instance;
-		var result: bool;
-		match (accessor.getOperand(0)) {
-			Ref(obj) => result = if(obj == null, nullable, Runtime.castObject(instance, ht_val, obj));
-			I31 => result = Runtime.castI31(instance, ht_val);
-			_ => result = false;
-		}
+		var result = Runtime.cast(instance, nullable, ht_val, accessor.getOperand(0));
 		if (result == success_taken) trace.pushNotTaken();
 		else trace.pushTaken();
 		return ProbeAction.Continue;