diff --git a/clang/lib/Format/UnwrappedLineParser.cpp b/clang/lib/Format/UnwrappedLineParser.cpp
--- a/clang/lib/Format/UnwrappedLineParser.cpp
+++ b/clang/lib/Format/UnwrappedLineParser.cpp
@@ -1212,9 +1212,13 @@
   // If a potential include guard has an #else, it's not an include guard.
   if (IncludeGuard == IG_Defined && PPBranchLevel == 0)
     IncludeGuard = IG_Rejected;
+  // Don't crash when there is an #else without an #if.
+  if (PPBranchLevel <= -1) {
+    conditionalCompilationStart(/*Unreachable=*/true);
+    assert(PPBranchLevel == 0);
+  }
   conditionalCompilationAlternative();
-  if (PPBranchLevel > -1)
-    --PPBranchLevel;
+  --PPBranchLevel;
   parsePPUnknown();
   ++PPBranchLevel;
 }
diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -5193,6 +5193,17 @@
   verifyNoCrash("a={0,1\n#if a\n#else\n;\n#endif\n}");
   verifyNoCrash("#if a\na(\n#else\n#endif\n) a {a,b,c,d,f,g};");
   verifyNoCrash("#ifdef A\n a(\n #else\n #endif\n) = []() {      \n)}");
+  std::function<void(std::string, unsigned)> FormatBadBranches =
+      [&](std::string Prefix, unsigned Lines) {
+        const std::string Directives[] = {"", "#if X\n", "#else X\n",
+                                          "#endif\n"};
+        if (Lines == 0)
+          verifyNoCrash(Prefix);
+        else
+          for (const auto &Part : Directives)
+            FormatBadBranches(Prefix + Part, Lines - 1);
+      };
+  FormatBadBranches("", 6);
 }
 
 TEST_F(FormatTest, MacrosWithoutTrailingSemicolon) {