diff --git a/src/extension/internal/pdfinput/pdf-parser.cpp b/src/extension/internal/pdfinput/pdf-parser.cpp
index c52675e..0b9ba15 100644
--- a/src/extension/internal/pdfinput/pdf-parser.cpp
+++ b/src/extension/internal/pdfinput/pdf-parser.cpp
@@ -2414,7 +2414,7 @@ void PdfParser::doShowText(GooString *s) {
   int wMode;
   double riseX, riseY;
   CharCode code;
-  Unicode *u = NULL;
+  const Unicode *u = NULL;
   double x, y, dx, dy, tdx, tdy;
   double originX, originY, tOriginX, tOriginY;
   double oldCTM[6], newCTM[6];

diff --git a/src/extension/internal/pdfinput/svg-builder.cpp b/src/extension/internal/pdfinput/svg-builder.cpp
index dd5e710..036fc70 100644
--- a/src/extension/internal/pdfinput/svg-builder.cpp
+++ b/src/extension/internal/pdfinput/svg-builder.cpp
@@ -1379,7 +1379,7 @@ void SvgBuilder::beginString(GfxState *state) {
 void SvgBuilder::addChar(GfxState *state, double x, double y,
                          double dx, double dy,
                          double originX, double originY,
-                         CharCode /*code*/, int /*nBytes*/, Unicode *u, int uLen) {
+                         CharCode /*code*/, int /*nBytes*/, const Unicode *u, int uLen) {
 
 
     bool is_space = ( uLen == 1 && u[0] == 32 );
diff --git a/src/extension/internal/pdfinput/svg-builder.h b/src/extension/internal/pdfinput/svg-builder.h
index 34b42e1..6fe62cc 100644
--- a/src/extension/internal/pdfinput/svg-builder.h
+++ b/src/extension/internal/pdfinput/svg-builder.h
@@ -141,7 +141,7 @@ public:
     void addChar(GfxState *state, double x, double y,
                  double dx, double dy,
                  double originX, double originY,
-                 CharCode code, int nBytes, Unicode *u, int uLen);
+                 CharCode code, int nBytes, const Unicode *u, int uLen);
     void beginTextObject(GfxState *state);
     void endTextObject(GfxState *state);