diff --git a/lib/ssd1306/ssd1306.c b/lib/ssd1306/ssd1306.c
--- a/lib/ssd1306/ssd1306.c
+++ b/lib/ssd1306/ssd1306.c
@@ -326,9 +326,9 @@ void ssd1306_drawlogo()
 // Print a single character
 void ssd1306_drawchar(unsigned char ascii, unsigned char row, unsigned char xPos)
 {
-    const unsigned char *srcPointer = (char*)-1;
+    const unsigned char *srcPointer = (unsigned char*)-1;
 
-    srcPointer = &fontData[(ascii-32)][0];
+    srcPointer = (unsigned char*)&fontData[(ascii-32)][0];
 
     setStartPage(row);
     setStartColumn(xPos);
@@ -407,7 +407,7 @@ void ssd1306_drawcharbig(unsigned char a
 
 
 // Print a string to the display
-void ssd1306_drawstring(const unsigned char *dataPtr, unsigned char row, unsigned char xPos)
+void ssd1306_drawstring(const char *dataPtr, unsigned char row, unsigned char xPos)
 {
     unsigned char *srcPointer;
 
@@ -425,7 +425,7 @@ void ssd1306_drawstring(const unsigned c
 
 
 // Print a string to the display, big font
-void ssd1306_drawstringbig(const unsigned char *dataPtr, unsigned char row, unsigned char xPos)
+void ssd1306_drawstringbig(const char *dataPtr, unsigned char row, unsigned char xPos)
 {
     char *srcPointer;