Index: lib/CodeGen/MIRParser/MIParser.cpp
===================================================================
--- lib/CodeGen/MIRParser/MIParser.cpp
+++ lib/CodeGen/MIRParser/MIParser.cpp
@@ -1303,11 +1303,11 @@
 }
 
 bool MIParser::parseLowLevelType(StringRef::iterator Loc, LLT &Ty) {
-  if (Token.range().front() == 's' || Token.range().front() == 'p')
-    if (!llvm::all_of(Token.range().drop_front(), isdigit))
+  if (Token.range().front() == 's' || Token.range().front() == 'p') {
+    StringRef SizeStr = Token.range().drop_front();
+    if (SizeStr.size() == 0 || !llvm::all_of(SizeStr, isdigit))
       return error("Expected integers after 's'/'p' type character");
-  if (!llvm::all_of(Token.range().drop_front(), isdigit))
-    return error("Expected integers after 's'/'p' type character");
+  }
 
   if (Token.range().front() == 's') {
     Ty = LLT::scalar(APSInt(Token.range().drop_front()).getZExtValue());
@@ -1339,7 +1339,8 @@
 
   if (Token.range().front() != 's')
     return error(Loc, "expected '<N x sM>' for vector type");
-  if (!llvm::all_of(Token.range().drop_front(), isdigit))
+  StringRef SizeStr = Token.range().drop_front();
+  if (SizeStr.size() == 0 || !llvm::all_of(SizeStr, isdigit))
     return error("Expected integers after 's' type character");
   uint64_t ScalarSize = APSInt(Token.range().drop_front()).getZExtValue();
   lex();
@@ -1354,7 +1355,13 @@
 
 bool MIParser::parseTypedImmediateOperand(MachineOperand &Dest) {
   assert(Token.is(MIToken::Identifier));
-  if (!llvm::all_of(Token.range().drop_front(), isdigit))
+  StringRef TypeStr = Token.range();
+  if (TypeStr.front() != 'i' && TypeStr.front() != 's' &&
+      TypeStr.front() != 'p')
+    return error(
+        "A typed immediate operand should start with one of 'i', 's', or 'p'");
+  StringRef SizeStr = Token.range().drop_front();
+  if (SizeStr.size() == 0 || !llvm::all_of(SizeStr, isdigit))
     return error("Expected integers after 'i'/'s'/'p' type character");
 
   auto Loc = Token.location();
Index: test/CodeGen/MIR/WebAssembly/typed-immediate-operand-invalid0.mir
===================================================================
--- /dev/null
+++ test/CodeGen/MIR/WebAssembly/typed-immediate-operand-invalid0.mir
@@ -0,0 +1,13 @@
+# RUN: not llc -mtriple=wasm32-unknown-unknown -run-pass none -o /dev/null %s 2>&1 | FileCheck %s
+# When a typed immediate operand is only a single 'i'/'s'/'p' character
+---
+name: test_typed_immediate_operand_invalid0
+liveins:
+  - { reg: '$arguments' }
+body: |
+  bb.0:
+    liveins: $arguments
+    ; CHECK: [[@LINE+1]]:24: Expected integers after 'i'/'s'/'p' type character
+    %0:i32 = CONST_I32 i 0, implicit-def dead $arguments
+    RETURN_VOID implicit-def dead $arguments
+...
Index: test/CodeGen/MIR/WebAssembly/typed-immediate-operand-invalid1.mir
===================================================================
--- /dev/null
+++ test/CodeGen/MIR/WebAssembly/typed-immediate-operand-invalid1.mir
@@ -0,0 +1,13 @@
+# RUN: not llc -mtriple=wasm32-unknown-unknown -run-pass none -o /dev/null %s 2>&1 | FileCheck %s
+# When a typed immediate operand does not start with 'i', 's', or 'p'
+---
+name: test_typed_immediate_operand_invalid1
+liveins:
+  - { reg: '$arguments' }
+body: |
+  bb.0:
+    liveins: $arguments
+    ; CHECK: [[@LINE+1]]:24: A typed immediate operand should start with one of 'i', 's', or 'p'
+    %0:i32 = CONST_I32 abc 0, implicit-def dead $arguments
+    RETURN_VOID implicit-def dead $arguments
+...