diff --git a/Sources/WasmParser/LEB.swift b/Sources/WasmParser/LEB.swift
index df99feec..7bbc0852 100644
--- a/Sources/WasmParser/LEB.swift
+++ b/Sources/WasmParser/LEB.swift
@@ -35,7 +35,7 @@ func decodeLEB128<IntType, Stream>(
 
 @inlinable
 func decodeLEB128<IntType, Stream>(
-    stream: Stream
+    stream: Stream, bitWidth: Int = IntType.bitWidth
 ) throws -> IntType where IntType: FixedWidthInteger, IntType: RawSignedInteger, Stream: ByteStream {
     let firstByte = try stream.consumeAny()
     var result = IntType.Unsigned(firstByte & 0b0111_1111)
@@ -54,8 +54,8 @@ func decodeLEB128<IntType, Stream>(
         result |= slice << shift
 
         // When we don't have enough bit width
-        if shift > (IntType.bitWidth - 7) {
-            let remainingBitWidth = IntType.bitWidth - Int(shift)
+        if shift > (bitWidth - 7) {
+            let remainingBitWidth = bitWidth - Int(shift)
             let continuationBit = (byte & 0b1000_0000) != 0
             // When a next byte is expected
             if continuationBit {
diff --git a/Sources/WasmParser/WasmParser.swift b/Sources/WasmParser/WasmParser.swift
index f171ec5a..db7bb815 100644
--- a/Sources/WasmParser/WasmParser.swift
+++ b/Sources/WasmParser/WasmParser.swift
@@ -316,6 +316,10 @@ extension WasmParserError.Message {
     @usableFromInline static func invalidResultArity(expected: Int, actual: Int) -> Self {
         Self("invalid result arity: expected \(expected) but got \(actual)")
     }
+
+    @usableFromInline static func invalidFunctionType(_ index: Int64) -> Self {
+        Self("invalid function type index: \(index), expected a unsigned 32-bit integer")
+    }
 }
 
 /// > Note:
@@ -344,6 +348,11 @@ extension ByteStream {
     func parseSigned<T: FixedWidthInteger & RawSignedInteger>() throws -> T {
         try decodeLEB128(stream: self)
     }
+
+    @usableFromInline
+    func parseVarSigned33() throws -> Int64 {
+        try decodeLEB128(stream: self, bitWidth: 33)
+    }
 }
 
 /// > Note:
@@ -452,7 +461,11 @@ extension Parser {
         case 0x7C...0x7F, 0x70, 0x6F:
             return try .type(parseValueType())
         default:
-            return try .funcType(TypeIndex(stream.consumeAny()))
+            let rawIndex = try stream.parseVarSigned33()
+            guard let index = TypeIndex(exactly: rawIndex) else {
+                throw makeError(.invalidFunctionType(rawIndex))
+            }
+            return .funcType(index)
         }
     }