diff --git a/libqpdf/Pl_PNGFilter.cc b/libqpdf/Pl_PNGFilter.cc
index 730d6d9..174e0b5 100644
--- a/libqpdf/Pl_PNGFilter.cc
+++ b/libqpdf/Pl_PNGFilter.cc
@@ -3,6 +3,7 @@
 #include <stdexcept>
 #include <string.h>
 #include <limits.h>
+#include <algorithm>
 
 Pl_PNGFilter::Pl_PNGFilter(char const* identifier, Pipeline* next,
 			   action_e action, unsigned int columns,
diff --git a/libqpdf/QUtil.cc b/libqpdf/QUtil.cc
index 1cc5449..2de1d76 100644
--- a/libqpdf/QUtil.cc
+++ b/libqpdf/QUtil.cc
@@ -326,7 +326,7 @@ QUtil::hex_decode(std::string const& input)
             }
             else
             {
-                result.back() += ch;
+                result[result.length()-1] += ch;
                 pos = 0;
             }
         }