diff --git a/llvm/lib/Support/RISCVISAInfo.cpp b/llvm/lib/Support/RISCVISAInfo.cpp
--- a/llvm/lib/Support/RISCVISAInfo.cpp
+++ b/llvm/lib/Support/RISCVISAInfo.cpp
@@ -622,6 +622,9 @@
     break;
   case 'g':
     // g = imafd
+    if (Arch.size() > 5 && isdigit(Arch[5]))
+      return createStringError(errc::invalid_argument,
+                               "version not supported for 'g'");
     StdExts = StdExts.drop_front(4);
     break;
   }
diff --git a/llvm/unittests/Support/RISCVISAInfoTest.cpp b/llvm/unittests/Support/RISCVISAInfoTest.cpp
--- a/llvm/unittests/Support/RISCVISAInfoTest.cpp
+++ b/llvm/unittests/Support/RISCVISAInfoTest.cpp
@@ -397,14 +397,11 @@
       "(this compiler supports 0.2)");
 }
 
-TEST(ParseArchString, AcceptsExtensionVersionForG) {
-  // FIXME: As there is no versioning scheme for G, arguably an error should
-  // be produced.
-  auto MaybeISAInfo = RISCVISAInfo::parseArchString("rv64g9p9", true, false);
-  ASSERT_THAT_EXPECTED(MaybeISAInfo, Succeeded());
-  RISCVISAInfo::OrderedExtensionMap Exts = (*MaybeISAInfo)->getExtensions();
-  EXPECT_EQ(Exts.size(), 5UL);
-  EXPECT_TRUE(Exts.at("i") == (RISCVExtensionInfo{"i", 2, 0}));
+TEST(ParseArchString, RejectsExtensionVersionForG) {
+  for (StringRef Input : {"rv32g1c", "rv64g2p0"}) {
+    EXPECT_EQ(toString(RISCVISAInfo::parseArchString(Input, true).takeError()),
+              "version not supported for 'g'");
+  }
 }
 
 TEST(ParseArchString, AddsImpliedExtensions) {