diff --git a/FlashMQTests/tst_maintests.cpp b/FlashMQTests/tst_maintests.cpp
index bf96ce4..996b739 100644
--- a/FlashMQTests/tst_maintests.cpp
+++ b/FlashMQTests/tst_maintests.cpp
@@ -78,6 +78,8 @@ private slots:
 
     void test_sse_split();
 
+    void test_validUtf8();
+
 };
 
 MainTests::MainTests()
@@ -608,6 +610,8 @@ void MainTests::test_sse_split()
     topics.push_back("//1234567890abcdef/1234567890abcdefg/koe/");
     topics.push_back("//1234567890abcdef/1234567890abcdefg/koe//");
     topics.push_back("//1234567890abcdef/1234567890abcdef/");
+    topics.push_back("/");
+    topics.push_back("");
 
     for (const std::string &t : topics)
     {
@@ -615,6 +619,69 @@ void MainTests::test_sse_split()
     }
 }
 
+void MainTests::test_validUtf8()
+{
+    char m[16];
+
+    QVERIFY(isValidUtf8(""));
+    QVERIFY(isValidUtf8("Hello"));
+
+    std::memset(m, 0, 16);
+    QVERIFY(!isValidUtf8(std::string(m, 16)));
+
+    QVERIFY(isValidUtf8("Straƀe")); // two byte chars
+    QVERIFY(isValidUtf8("StraƀeHelloHelloHelloHelloHelloHello")); // two byte chars
+    QVERIFY(isValidUtf8("HelloHelloHelloHelloHelloHelloHelloHelloStraƀeHelloHelloHelloHelloHelloHello")); // two byte chars
+
+    std::memset(m, 0, 16);
+    m[0] = 'a';
+    m[1] = 13; // is \r
+    QVERIFY(!isValidUtf8(std::string(m, 16)));
+
+    const std::string unicode_ballet_shoes("🩰");
+    QVERIFY(unicode_ballet_shoes.length() == 4);
+    QVERIFY(isValidUtf8(unicode_ballet_shoes));
+
+    const std::string unicode_ballot_box("☐");
+    QVERIFY(unicode_ballot_box.length() == 3);
+    QVERIFY(isValidUtf8(unicode_ballot_box));
+
+    std::memset(m, 0, 16);
+    m[0] = 0b11000001; // Start 2 byte char
+    m[1] = 0b00000001; // Next byte doesn't start with 1, which is wrong
+    std::string a(m, 2);
+    QVERIFY(!isValidUtf8(a));
+
+    std::memset(m, 0, 16);
+    m[0] = 0b11100001; // Start 3 byte char
+    m[1] = 0b10100001;
+    m[2] = 0b00000001; // Next byte doesn't start with 1, which is wrong
+    std::string b(m, 3);
+    QVERIFY(!isValidUtf8(b));
+
+    std::memset(m, 0, 16);
+    m[0] = 0b11110001; // Start 4 byte char
+    m[1] = 0b10100001;
+    m[2] = 0b10100001;
+    m[3] = 0b00000001; // Next byte doesn't start with 1, which is wrong
+    std::string c(m, 4);
+    QVERIFY(!isValidUtf8(c));
+
+    std::memset(m, 0, 16);
+    m[0] = 0b11110001; // Start 4 byte char
+    m[1] = 0b10100001;
+    m[2] = 0b00100001; // Doesn't start with 1: invalid.
+    m[3] = 0b10000001;
+    std::string d(m, 4);
+    QVERIFY(!isValidUtf8(d));
+
+    // Upper ASCII, invalid
+    std::memset(m, 0, 16);
+    m[0] = 127;
+    std::string e(m, 1);
+    QVERIFY(!isValidUtf8(e));
+}
+
 QTEST_GUILESS_MAIN(MainTests)
 
 #include "tst_maintests.moc"
diff --git a/threaddata.cpp b/threaddata.cpp
index d231724..9b56c7b 100644
--- a/threaddata.cpp
+++ b/threaddata.cpp
@@ -18,6 +18,7 @@ License along with FlashMQ. If not, see <https://www.gnu.org/licenses/>.
 #include "threaddata.h"
 #include <string>
 #include <sstream>
+#include <cassert>
 
 #define TOPIC_MEMORY_LENGTH 65560
 
@@ -181,6 +182,7 @@ std::vector<std::string> *ThreadData::splitTopic(const std::string &topic)
         __m128i loaded = _mm_loadu_si128((__m128i*)i);
 
         int len_left = s - n;
+        assert(len_left >= 0);
         int index = _mm_cmpestri(slashes, 1, loaded, len_left, 0);
         std::memcpy(&subtopicParseMem[carryi], i, index);
         carryi += std::min<int>(index, len_left);
diff --git a/utils.cpp b/utils.cpp
index bdb9a3d..92b6832 100644
--- a/utils.cpp
+++ b/utils.cpp
@@ -83,11 +83,8 @@ bool isValidUtf8(const std::string &s, bool alsoCheckInvalidPublishChars)
 {
     int multibyte_remain = 0;
     int cur_code_point = 0;
-    for(const char &x : s)
+    for(const char x : s)
     {
-        if (x == 0)
-            return false;
-
         if (alsoCheckInvalidPublishChars && (x == '#' || x == '+'))
             return false;
 
@@ -95,7 +92,11 @@ bool isValidUtf8(const std::string &s, bool alsoCheckInvalidPublishChars)
         {
             cur_code_point = 0;
 
-            if((x & 0b11100000) == 0b11000000) // 2 byte char
+            if ((x & 0b10000000) == 0) // when the MSB is 0, it's ASCII, most common case
+            {
+                cur_code_point += (x & 0b01111111);
+            }
+            else if((x & 0b11100000) == 0b11000000) // 2 byte char
             {
                 multibyte_remain = 1;
                 cur_code_point += ((x & 0b00011111) << 6);
@@ -128,7 +129,7 @@ bool isValidUtf8(const std::string &s, bool alsoCheckInvalidPublishChars)
             // Invalid range for MQTT. [MQTT-1.5.3-1]
             if (cur_code_point >= 0xD800 && cur_code_point <= 0xDFFF) // Dec 55296-57343
                 return false;
-            if (cur_code_point >= 0x0001 && cur_code_point <= 0x001F)
+            if (cur_code_point <= 0x001F)
                 return false;
             if (cur_code_point >= 0x007F && cur_code_point <= 0x009F)
                 return false;