diff --git a/include/rapidjson/reader.h b/include/rapidjson/reader.h
index d92d9fb4..00ab6a5f 100644
--- a/include/rapidjson/reader.h
+++ b/include/rapidjson/reader.h
@@ -1247,18 +1247,27 @@ private:
         }
         // Parse NaN or Infinity here
         else if ((parseFlags & kParseNanAndInfFlag) && RAPIDJSON_LIKELY((s.Peek() == 'I' || s.Peek() == 'N'))) {
-            useNanOrInf = true;
-            if (RAPIDJSON_LIKELY(Consume(s, 'N') && Consume(s, 'a') && Consume(s, 'N'))) {
-                d = std::numeric_limits<double>::quiet_NaN();
+            if (Consume(s, 'N')) {
+                if (Consume(s, 'a') && Consume(s, 'N')) {
+                    d = std::numeric_limits<double>::quiet_NaN();
+                    useNanOrInf = true;
+                }
             }
-            else if (RAPIDJSON_LIKELY(Consume(s, 'I') && Consume(s, 'n') && Consume(s, 'f'))) {
-                d = (minus ? -std::numeric_limits<double>::infinity() : std::numeric_limits<double>::infinity());
-                if (RAPIDJSON_UNLIKELY(s.Peek() == 'i' && !(Consume(s, 'i') && Consume(s, 'n')
-                                                            && Consume(s, 'i') && Consume(s, 't') && Consume(s, 'y'))))
-                    RAPIDJSON_PARSE_ERROR(kParseErrorValueInvalid, s.Tell());
+            else if (RAPIDJSON_LIKELY(Consume(s, 'I'))) {
+                if (Consume(s, 'n') && Consume(s, 'f')) {
+                    d = (minus ? -std::numeric_limits<double>::infinity() : std::numeric_limits<double>::infinity());
+                    useNanOrInf = true;
+
+                    if (RAPIDJSON_UNLIKELY(s.Peek() == 'i' && !(Consume(s, 'i') && Consume(s, 'n')
+                                                                && Consume(s, 'i') && Consume(s, 't') && Consume(s, 'y')))) {
+                        RAPIDJSON_PARSE_ERROR(kParseErrorValueInvalid, s.Tell());
+                    }
+                }
             }
-            else
+            
+            if (RAPIDJSON_UNLIKELY(!useNanOrInf)) {
                 RAPIDJSON_PARSE_ERROR(kParseErrorValueInvalid, s.Tell());
+            }
         }
         else
             RAPIDJSON_PARSE_ERROR(kParseErrorValueInvalid, s.Tell());
diff --git a/test/unittest/readertest.cpp b/test/unittest/readertest.cpp
index b1c0c31c..3555f11a 100644
--- a/test/unittest/readertest.cpp
+++ b/test/unittest/readertest.cpp
@@ -1877,6 +1877,10 @@ TEST(Reader, ParseNanAndInfinity) {
     TEST_NAN_INF("Infinity", inf);
     TEST_NAN_INF("-Inf", -inf);
     TEST_NAN_INF("-Infinity", -inf);
+    TEST_NAN_INF_ERROR(kParseErrorValueInvalid, "NInf", 1);
+    TEST_NAN_INF_ERROR(kParseErrorValueInvalid, "NaInf", 2);
+    TEST_NAN_INF_ERROR(kParseErrorValueInvalid, "INan", 1);
+    TEST_NAN_INF_ERROR(kParseErrorValueInvalid, "InNan", 2);
     TEST_NAN_INF_ERROR(kParseErrorValueInvalid, "nan", 1);
     TEST_NAN_INF_ERROR(kParseErrorValueInvalid, "-nan", 1);
     TEST_NAN_INF_ERROR(kParseErrorValueInvalid, "NAN", 1);