diff --git a/apps/code/python_text_area.cpp b/apps/code/python_text_area.cpp
index b38700d23..c7e958aab 100644
--- a/apps/code/python_text_area.cpp
+++ b/apps/code/python_text_area.cpp
@@ -119,22 +119,20 @@ void PythonTextArea::ContentView::drawLine(KDContext * ctx, int line, const char
      * basis. This can work, however the MicroPython lexer won't accept a line
      * starting with a whitespace. So we're discarding leading whitespaces
      * beforehand. */
-    UTF8Decoder decoder(text);
-    const char * p = decoder.stringPosition();
-    CodePoint c = decoder.nextCodePoint();
-    while (p < text + byteLength && c  == ' ') {
-        p = decoder.stringPosition();
-        c = decoder.nextCodePoint();
+    const char * firstNonSpace = UTF8Helper::NotCodePointSearch(text, ' ');
+    if (UTF8Helper::CodePointIs(firstNonSpace, UCodePointNull)) {
+      nlr_pop();
+      return;
     }
 
-    mp_lexer_t * lex = mp_lexer_new_from_str_len(0, p, byteLength - (p - text), 0);
+    mp_lexer_t * lex = mp_lexer_new_from_str_len(0, firstNonSpace, byteLength - (firstNonSpace - text), 0);
     LOG_DRAW("Pop token %d\n", lex->tok_kind);
 
-    const char * tokenFrom = p;
+    const char * tokenFrom = firstNonSpace;
     size_t tokenLength = 0;
     while (lex->tok_kind != MP_TOKEN_NEWLINE && lex->tok_kind != MP_TOKEN_END) {
 
-      tokenFrom = p + lex->tok_column - 1;
+      tokenFrom = firstNonSpace + lex->tok_column - 1;
       tokenLength = TokenLength(lex);
       LOG_DRAW("Draw \"%.*s\" for token %d\n", tokenLength, tokenFrom, lex->tok_kind);
       drawStringAt(ctx, line,
@@ -151,7 +149,7 @@ void PythonTextArea::ContentView::drawLine(KDContext * ctx, int line, const char
 
     tokenFrom += tokenLength;
     if (tokenFrom < text + byteLength) {
-      LOG_DRAW("Draw comment \"%.*s\" from %d\n", byteLength - (tokenFrom - text), p, tokenFrom);
+      LOG_DRAW("Draw comment \"%.*s\" from %d\n", byteLength - (tokenFrom - text), firstNonSpace, tokenFrom);
       drawStringAt(ctx, line,
           UTF8Helper::GlyphOffsetAtCodePoint(text, tokenFrom),
           tokenFrom,
diff --git a/escher/src/text_area.cpp b/escher/src/text_area.cpp
index 9e41714d9..3478e06ba 100644
--- a/escher/src/text_area.cpp
+++ b/escher/src/text_area.cpp
@@ -57,10 +57,10 @@ bool TextArea::handleEvent(Ion::Events::Event event) {
     decoder.previousCodePoint();
     return setCursorLocation(decoder.stringPosition());
   } else if (event == Ion::Events::Right) {
-    if (*cursorLocation() == 0) {
+    if (UTF8Helper::CodePointIs(cursorLocation(), UCodePointNull)) {
       return false;
     }
-    UTF8Decoder decoder(text(), cursorLocation());
+    UTF8Decoder decoder(cursorLocation());
     decoder.nextCodePoint();
     return setCursorLocation(decoder.stringPosition());
   } else if (event == Ion::Events::Up) {
@@ -216,7 +216,9 @@ CodePoint TextArea::Text::removeCodePoint(char * * position) {
 
 size_t TextArea::Text::removeRemainingLine(const char * location, int direction) {
   assert(m_buffer != nullptr);
-  assert(location >= m_buffer && location < m_buffer + m_bufferSize);
+  assert(location >= m_buffer && location <= m_buffer + m_bufferSize);
+  assert(direction > 0 || location > m_buffer);
+  assert(direction < 0 || location < m_buffer + m_bufferSize);
 
   UTF8Decoder decoder(m_buffer, location);
   const char * codePointPosition = decoder.stringPosition();
diff --git a/escher/src/text_field.cpp b/escher/src/text_field.cpp
index 2ee9ef894..0ef6931e6 100644
--- a/escher/src/text_field.cpp
+++ b/escher/src/text_field.cpp
@@ -440,7 +440,7 @@ bool TextField::privateHandleMoveEvent(Ion::Events::Event event) {
   }
   if (event == Ion::Events::Right && isEditing() && cursorLocation() < m_contentView.draftTextBuffer() + draftTextLength()) {
     assert(isEditing());
-    UTF8Decoder decoder(m_contentView.draftTextBuffer(), cursorLocation());
+    UTF8Decoder decoder(cursorLocation());
     decoder.nextCodePoint();
     return setCursorLocation(decoder.stringPosition());
   }
diff --git a/ion/include/ion/unicode/utf8_helper.h b/ion/include/ion/unicode/utf8_helper.h
index 4a7f27985..337c38f96 100644
--- a/ion/include/ion/unicode/utf8_helper.h
+++ b/ion/include/ion/unicode/utf8_helper.h
@@ -15,7 +15,7 @@ const char * CodePointSearch(const char * s, CodePoint c);
 
 /* Returns the first occurence of a code point that is not c in a string,
  * stopping at the null-terminating char or the start of string. */
-const char * NotCodePointSearch(const char * s, CodePoint c, bool goingLeft, const char * initialPosition);
+const char * NotCodePointSearch(const char * s, CodePoint c, bool goingLeft = false, const char * initialPosition = nullptr);
 
 /* Copy src into dst while removing all code points c. Also update an index
  * that should be lower if code points where removed before it. Ensure null-
diff --git a/ion/src/shared/unicode/utf8_decoder.cpp b/ion/src/shared/unicode/utf8_decoder.cpp
index c95279bb1..45e92ad49 100644
--- a/ion/src/shared/unicode/utf8_decoder.cpp
+++ b/ion/src/shared/unicode/utf8_decoder.cpp
@@ -4,6 +4,7 @@
 static inline int leading_ones(uint8_t value) {
   for (int i=0; i<8; i++) {
     if (!(value & 0x80)) {
+      assert(i <= 4);
       return i;
     }
     value = value << 1;
@@ -17,7 +18,7 @@ static inline uint8_t last_k_bits(uint8_t value, uint8_t bits) {
 }
 
 CodePoint UTF8Decoder::nextCodePoint() {
-  assert(m_stringPosition == m_stringPosition || *(m_stringPosition - 1) != 0);
+  assert(m_stringPosition == m_string || *(m_stringPosition - 1) != 0);
   int leadingOnes = leading_ones(*m_stringPosition);
   uint32_t result = last_k_bits(*m_stringPosition++, 8-leadingOnes-1);
   for (int i = 0; i < leadingOnes - 1; i++) {
diff --git a/ion/src/shared/unicode/utf8_helper.cpp b/ion/src/shared/unicode/utf8_helper.cpp
index 2123229bc..384db546b 100644
--- a/ion/src/shared/unicode/utf8_helper.cpp
+++ b/ion/src/shared/unicode/utf8_helper.cpp
@@ -9,6 +9,7 @@ static inline int minInt(int x, int y) { return x < y ? x : y; }
 static inline size_t minSizeT(size_t x, size_t y) { return x < y ? x : y; }
 
 int CountOccurrences(const char * s, CodePoint c) {
+  assert(c != UCodePointNull);
   int count = 0;
   if (UTF8Decoder::CharSizeOfCodePoint(c) == 1) {
     /* The code point is one char long, so it is equal to its char translation.
@@ -55,6 +56,7 @@ const char * CodePointSearch(const char * s, CodePoint c) {
 }
 
 const char * NotCodePointSearch(const char * s, CodePoint c, bool goingLeft, const char * initialPosition) {
+  // TODO LEA: optimize for one byte long c?
   if (goingLeft) {
     assert(initialPosition != nullptr);
     if (initialPosition == s) {
diff --git a/poincare/src/parsing/tokenizer.cpp b/poincare/src/parsing/tokenizer.cpp
index 878e442ec..10ec5ef36 100644
--- a/poincare/src/parsing/tokenizer.cpp
+++ b/poincare/src/parsing/tokenizer.cpp
@@ -15,19 +15,13 @@ static inline bool isDigit(const CodePoint c) {
 
 const CodePoint Tokenizer::nextCodePoint(PopTest popTest, CodePoint context, bool * testResult) {
   UTF8Decoder decoder(m_text);
-  const char * currentPointer = m_text;
   CodePoint firstCodePoint = decoder.nextCodePoint();
-  const char * nextPointer = decoder.stringPosition();
-  size_t numberOfBytesForCodePoint = nextPointer - currentPointer;
+  const char * nextTextPosition = decoder.stringPosition();
   if (firstCodePoint != UCodePointNull) {
-    currentPointer = nextPointer;
     CodePoint codePoint = decoder.nextCodePoint();
-    nextPointer = decoder.stringPosition();
     while (codePoint.isCombining()) {
-      numberOfBytesForCodePoint+= nextPointer - currentPointer;
-      currentPointer = nextPointer;
+      nextTextPosition = decoder.stringPosition();
       codePoint = decoder.nextCodePoint();
-      nextPointer = decoder.stringPosition();
     }
   }
   // TODO handle combined code points? For now the combining codepoints get dropped.
@@ -36,7 +30,7 @@ const CodePoint Tokenizer::nextCodePoint(PopTest popTest, CodePoint context, boo
     *testResult = shouldPop;
   }
   if (shouldPop) {
-    m_text+= numberOfBytesForCodePoint;
+    m_text = nextTextPosition;
   }
   return firstCodePoint;
 }