diff --git a/src/libexpr/lexer.l b/src/libexpr/lexer.l
index 29ca327c1..a052447d3 100644
--- a/src/libexpr/lexer.l
+++ b/src/libexpr/lexer.l
@@ -12,6 +12,8 @@
 
 
 %{
+#include <boost/lexical_cast.hpp>
+
 #include "nixexpr.hh"
 #include "parser-tab.hh"
 
@@ -124,9 +126,11 @@ or          { return OR_KW; }
 
 {ID}        { yylval->id = strdup(yytext); return ID; }
 {INT}       { errno = 0;
-              yylval->n = strtol(yytext, 0, 10);
-              if (errno != 0)
+              try {
+                  yylval->n = boost::lexical_cast<int64_t>(yytext);
+              } catch (const boost::bad_lexical_cast &) {
                   throw ParseError(format("invalid integer '%1%'") % yytext);
+              }
               return INT;
             }
 {FLOAT}     { errno = 0;
diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index 809772f7c..e1ec87d3b 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -43,7 +43,7 @@ class XMLWriter;
 class JSONPlaceholder;
 
 
-typedef long NixInt;
+typedef int64_t NixInt;
 typedef double NixFloat;
 
 /* External values must descend from ExternalValueBase, so that