Index: lib/asan/asan_flags.cc
===================================================================
--- lib/asan/asan_flags.cc
+++ lib/asan/asan_flags.cc
@@ -120,12 +120,12 @@
 #endif
 
   // Override from command line.
-  asan_parser.ParseString(GetEnv("ASAN_OPTIONS"));
+  asan_parser.ParseString(GetEnv("ASAN_OPTIONS"), "ASAN_OPTIONS");
 #if CAN_SANITIZE_LEAKS
-  lsan_parser.ParseString(GetEnv("LSAN_OPTIONS"));
+  lsan_parser.ParseString(GetEnv("LSAN_OPTIONS"), "LSAN_OPTIONS");
 #endif
 #if CAN_SANITIZE_UB
-  ubsan_parser.ParseString(GetEnv("UBSAN_OPTIONS"));
+  ubsan_parser.ParseString(GetEnv("UBSAN_OPTIONS"), "UBSAN_OPTIONS");
 #endif
 
   InitializeCommonFlags();
Index: lib/lsan/lsan.cc
===================================================================
--- lib/lsan/lsan.cc
+++ lib/lsan/lsan.cc
@@ -76,7 +76,7 @@
   // Override from user-specified string.
   const char *lsan_default_options = MaybeCallLsanDefaultOptions();
   parser.ParseString(lsan_default_options);
-  parser.ParseString(GetEnv("LSAN_OPTIONS"));
+  parser.ParseString(GetEnv("LSAN_OPTIONS"), "LSAN_OPTIONS");
 
   SetVerbosity(common_flags()->verbosity);
 
Index: lib/sanitizer_common/sanitizer_flag_parser.h
===================================================================
--- lib/sanitizer_common/sanitizer_flag_parser.h
+++ lib/sanitizer_common/sanitizer_flag_parser.h
@@ -112,7 +112,7 @@
   FlagParser();
   void RegisterHandler(const char *name, FlagHandlerBase *handler,
                        const char *desc);
-  void ParseString(const char *s);
+  void ParseString(const char *s, const char *env_option_name = 0);
   bool ParseFile(const char *path, bool ignore_missing);
   void PrintFlagDescriptions();
 
@@ -122,8 +122,8 @@
   void fatal_error(const char *err);
   bool is_space(char c);
   void skip_whitespace();
-  void parse_flags();
-  void parse_flag();
+  void parse_flags(const char *env_option_name);
+  void parse_flag(const char *env_option_name);
   bool run_handler(const char *name, const char *value);
   char *ll_strndup(const char *s, uptr n);
 };
Index: lib/sanitizer_common/sanitizer_flag_parser.cc
===================================================================
--- lib/sanitizer_common/sanitizer_flag_parser.cc
+++ lib/sanitizer_common/sanitizer_flag_parser.cc
@@ -75,10 +75,17 @@
   while (is_space(buf_[pos_])) ++pos_;
 }
 
-void FlagParser::parse_flag() {
+void FlagParser::parse_flag(const char *env_option_name) {
   uptr name_start = pos_;
   while (buf_[pos_] != 0 && buf_[pos_] != '=' && !is_space(buf_[pos_])) ++pos_;
-  if (buf_[pos_] != '=') fatal_error("expected '='");
+  if (buf_[pos_] != '=') {
+    if (env_option_name) {
+      Printf("ERROR: expected '=' in %s\n", env_option_name);
+      Die();
+    }
+    else
+      fatal_error("expected '='");
+  }
   char *name = ll_strndup(buf_ + name_start, pos_ - name_start);
 
   uptr value_start = ++pos_;
@@ -100,11 +107,11 @@
   if (!res) fatal_error("Flag parsing failed.");
 }
 
-void FlagParser::parse_flags() {
+void FlagParser::parse_flags(const char *env_option_name) {
   while (true) {
     skip_whitespace();
     if (buf_[pos_] == 0) break;
-    parse_flag();
+    parse_flag(env_option_name);
   }
 
   // Do a sanity check for certain flags.
@@ -112,7 +119,7 @@
     common_flags_dont_use.malloc_context_size = 1;
 }
 
-void FlagParser::ParseString(const char *s) {
+void FlagParser::ParseString(const char *s, const char *env_option_name) {
   if (!s) return;
   // Backup current parser state to allow nested ParseString() calls.
   const char *old_buf_ = buf_;
@@ -120,7 +127,7 @@
   buf_ = s;
   pos_ = 0;
 
-  parse_flags();
+  parse_flags(env_option_name);
 
   buf_ = old_buf_;
   pos_ = old_pos_;
Index: lib/tsan/rtl/tsan_flags.h
===================================================================
--- lib/tsan/rtl/tsan_flags.h
+++ lib/tsan/rtl/tsan_flags.h
@@ -27,7 +27,7 @@
   void ParseFromString(const char *str);
 };
 
-void InitializeFlags(Flags *flags, const char *env);
+void InitializeFlags(Flags *flags, const char *env, const char *env_option_name);
 }  // namespace __tsan
 
 #endif  // TSAN_FLAGS_H
Index: lib/tsan/rtl/tsan_flags.cc
===================================================================
--- lib/tsan/rtl/tsan_flags.cc
+++ lib/tsan/rtl/tsan_flags.cc
@@ -49,7 +49,7 @@
       &f->second_deadlock_stack);
 }
 
-void InitializeFlags(Flags *f, const char *env) {
+void InitializeFlags(Flags *f, const char *env, const char *env_option_name) {
   SetCommonFlagsDefaults();
   {
     // Override some common flags defaults.
@@ -91,9 +91,9 @@
   ubsan_parser.ParseString(ubsan_default_options);
 #endif
   // Override from command line.
-  parser.ParseString(env);
+  parser.ParseString(env, env_option_name);
 #if TSAN_CONTAINS_UBSAN
-  ubsan_parser.ParseString(GetEnv("UBSAN_OPTIONS"));
+  ubsan_parser.ParseString(GetEnv("UBSAN_OPTIONS"), "UBSAN_OPTIONS");
 #endif
 
   // Sanity check.
Index: lib/tsan/rtl/tsan_rtl.cc
===================================================================
--- lib/tsan/rtl/tsan_rtl.cc
+++ lib/tsan/rtl/tsan_rtl.cc
@@ -350,10 +350,11 @@
   SetCheckFailedCallback(TsanCheckFailed);
 
   ctx = new(ctx_placeholder) Context;
-  const char *options = GetEnv(SANITIZER_GO ? "GORACE" : "TSAN_OPTIONS");
+  const char *env_name = SANITIZER_GO ? "GORACE" : "TSAN_OPTIONS";
+  const char *options = GetEnv(env_name);
   CacheBinaryName();
   CheckASLR();
-  InitializeFlags(&ctx->flags, options);
+  InitializeFlags(&ctx->flags, options, env_name);
   AvoidCVE_2016_2143();
   __sanitizer::InitializePlatformEarly();
   __tsan::InitializePlatformEarly();
Index: lib/ubsan/ubsan_flags.cc
===================================================================
--- lib/ubsan/ubsan_flags.cc
+++ lib/ubsan/ubsan_flags.cc
@@ -69,7 +69,7 @@
   // Override from user-specified string.
   parser.ParseString(MaybeCallUbsanDefaultOptions());
   // Override from environment variable.
-  parser.ParseString(GetFlag("UBSAN_OPTIONS"));
+  parser.ParseString(GetFlag("UBSAN_OPTIONS"), "UBSAN_OPTIONS");
   InitializeCommonFlags();
   if (Verbosity()) ReportUnrecognizedFlags();