diff --git a/tools/inspector_protocol/encoding/encoding.cc b/tools/inspector_protocol/encoding/encoding.cc
index 89988476b87851..f7e933e41afac5 100644
--- a/tools/inspector_protocol/encoding/encoding.cc
+++ b/tools/inspector_protocol/encoding/encoding.cc
@@ -1386,7 +1386,7 @@ class JSONEncoder : public StreamingParserHandler {
         // Disallow overlong encodings for ascii characters, as these
         // would include " and other characters significant to JSON
         // string termination / control.
-        if (codepoint < 0x7f)
+        if (codepoint <= 0x7f)
           continue;
         // Invalid in UTF8, and can't be represented in UTF16 anyway.
         if (codepoint > 0x10ffff)
diff --git a/tools/inspector_protocol/encoding/encoding_test.cc b/tools/inspector_protocol/encoding/encoding_test.cc
index 6893fe2581683c..554aa1b2cbaedb 100644
--- a/tools/inspector_protocol/encoding/encoding_test.cc
+++ b/tools/inspector_protocol/encoding/encoding_test.cc
@@ -1325,6 +1325,25 @@ void WriteUTF8AsUTF16(StreamingParserHandler* writer, const std::string& utf8) {
   writer->HandleString16(SpanFrom(UTF8ToUTF16(SpanFrom(utf8))));
 }
 
+TEST(JsonEncoder, OverlongEncodings) {
+  std::string out;
+  Status status;
+  std::unique_ptr<StreamingParserHandler> writer =
+      NewJSONEncoder(&GetTestPlatform(), &out, &status);
+
+  // We encode 0x7f, which is the DEL ascii character, as a 4 byte UTF8
+  // sequence. This is called an overlong encoding, because only 1 byte
+  // is needed to represent 0x7f as UTF8.
+  std::vector<uint8_t> chars = {
+      0xf0,  // Starts 4 byte utf8 sequence
+      0x80,  // continuation byte
+      0x81,  // continuation byte w/ payload bit 7 set to 1.
+      0xbf,  // continuation byte w/ payload bits 0-6 set to 11111.
+  };
+  writer->HandleString8(SpanFrom(chars));
+  EXPECT_EQ("\"\"", out);  // Empty string means that 0x7f was rejected (good).
+}
+
 TEST(JsonStdStringWriterTest, HelloWorld) {
   std::string out;
   Status status;
@@ -1561,6 +1580,13 @@ TEST_F(JsonParserTest, UsAsciiDelCornerCase) {
       "string16: a\x7f\n"
       "map end\n",
       log_.str());
+
+  // We've seen an implementation of UTF16ToUTF8 which would replace the DEL
+  // character with ' ', so this simple roundtrip tests the routines in
+  // encoding_test_helper.h, to make test failures of the above easier to
+  // diagnose.
+  std::vector<uint16_t> utf16 = UTF8ToUTF16(SpanFrom(json));
+  EXPECT_EQ(json, UTF16ToUTF8(SpanFrom(utf16)));
 }
 
 TEST_F(JsonParserTest, Whitespace) {
diff --git a/tools/inspector_protocol/lib/encoding_cpp.template b/tools/inspector_protocol/lib/encoding_cpp.template
index 8a9d6f64e471f8..a0377d12f7dbb1 100644
--- a/tools/inspector_protocol/lib/encoding_cpp.template
+++ b/tools/inspector_protocol/lib/encoding_cpp.template
@@ -1394,7 +1394,7 @@ class JSONEncoder : public StreamingParserHandler {
         // Disallow overlong encodings for ascii characters, as these
         // would include " and other characters significant to JSON
         // string termination / control.
-        if (codepoint < 0x7f)
+        if (codepoint <= 0x7f)
           continue;
         // Invalid in UTF8, and can't be represented in UTF16 anyway.
         if (codepoint > 0x10ffff)