diff --git a/SSD1306Ascii.cpp b/SSD1306Ascii.cpp
index bc13270..342ec75 100644
--- a/SSD1306Ascii.cpp
+++ b/SSD1306Ascii.cpp
@@ -73,8 +73,8 @@ void SSD1306Ascii::setCursor(uint8_t col, uint8_t row) {
 //------------------------------------------------------------------------------
 void SSD1306Ascii::setFont(const uint8_t* font) {
   m_font = font;
-  fontFirstChar = readFontByte(m_font + FONT_FIRST_CHAR);
-  fontCharCount = readFontByte(m_font + FONT_CHAR_COUNT); 
+  m_fontFirstChar = readFontByte(m_font + FONT_FIRST_CHAR);
+  m_fontCharCount = readFontByte(m_font + FONT_CHAR_COUNT); 
 }
 //------------------------------------------------------------------------------
 void SSD1306Ascii::setRow(uint8_t row) {
@@ -104,8 +104,8 @@ size_t SSD1306Ascii::write(uint8_t ch) {
   }
   const uint8_t* base = m_font + FONT_WIDTH_TABLE;
 
-  if (ch < fontFirstChar || ch >= (fontFirstChar + fontCharCount)) return 0;
-  ch -= fontFirstChar;
+  if (ch < m_fontFirstChar || ch >= (m_fontFirstChar + m_fontCharCount)) return 0;
+  ch -= m_fontFirstChar;
   base += fontWidth * ch;
   for (uint8_t c = 0; c < fontWidth; c++) {
     uint8_t b = readFontByte(base + c);
diff --git a/SSD1306Ascii.h b/SSD1306Ascii.h
index 84dc390..60bf1b7 100644
--- a/SSD1306Ascii.h
+++ b/SSD1306Ascii.h
@@ -200,8 +200,8 @@ class SSD1306Ascii : public Print {
   const int fontWidth = 5;
   const int fontHeight = 7;
   const uint8_t letterSpacing = 1;
-  uint8_t fontFirstChar;
-  uint8_t fontCharCount;
+  uint8_t m_fontFirstChar;
+  uint8_t m_fontCharCount;
 };