diff --git a/lldb/source/Utility/FileSpec.cpp b/lldb/source/Utility/FileSpec.cpp
--- a/lldb/source/Utility/FileSpec.cpp
+++ b/lldb/source/Utility/FileSpec.cpp
@@ -310,7 +310,7 @@
     return Style::posix;
   if (absolute_path.startswith(R"(\\)"))
     return Style::windows;
-  if (absolute_path.size() > 3 && llvm::isAlpha(absolute_path[0]) &&
+  if (absolute_path.size() >= 3 && llvm::isAlpha(absolute_path[0]) &&
       absolute_path.substr(1, 2) == R"(:\)")
     return Style::windows;
   return llvm::None;
diff --git a/lldb/unittests/Utility/FileSpecTest.cpp b/lldb/unittests/Utility/FileSpecTest.cpp
--- a/lldb/unittests/Utility/FileSpecTest.cpp
+++ b/lldb/unittests/Utility/FileSpecTest.cpp
@@ -198,8 +198,10 @@
             FileSpec::GuessPathStyle(R"(C:\foo.txt)"));
   EXPECT_EQ(FileSpec::Style::windows,
             FileSpec::GuessPathStyle(R"(\\net\foo.txt)"));
+  EXPECT_EQ(FileSpec::Style::windows, FileSpec::GuessPathStyle(R"(Z:\)"));
   EXPECT_EQ(llvm::None, FileSpec::GuessPathStyle("foo.txt"));
   EXPECT_EQ(llvm::None, FileSpec::GuessPathStyle("foo/bar.txt"));
+  EXPECT_EQ(llvm::None, FileSpec::GuessPathStyle("Z:"));
 }
 
 TEST(FileSpecTest, GetPath) {