diff --git a/Sources/TOMLDecoder/Parsing/Parser.swift b/Sources/TOMLDecoder/Parsing/Parser.swift
index 730006c..c0e393b 100644
--- a/Sources/TOMLDecoder/Parsing/Parser.swift
+++ b/Sources/TOMLDecoder/Parsing/Parser.swift
@@ -1100,8 +1100,6 @@ extension Token {
         var index = text.lowerBound
         var endIndex = text.upperBound
 
-        assert(quoteChar == CodeUnits.doubleQuote || quoteChar == CodeUnits.singleQuote)
-
         if endIndex - index >= 3 && bytes[index] == CodeUnits.doubleQuote && bytes[index + 1] == CodeUnits.doubleQuote && bytes[index + 2] == CodeUnits.doubleQuote ||
             endIndex - index >= 3 && bytes[index] == CodeUnits.singleQuote && bytes[index + 1] == CodeUnits.singleQuote && bytes[index + 2] == CodeUnits.singleQuote
         {
@@ -1117,7 +1115,7 @@ extension Token {
         } else {
             index = index + 1
             endIndex = endIndex - 1
-            guard bytes[endIndex] == quoteChar else {
+            guard endIndex >= 0, bytes[endIndex] == quoteChar else {
                 throw TOMLError(.invalidString(context: context, lineNumber: lineNumber, reason: "missing closing quote"))
             }
         }