diff --git a/src/core/rational.cc b/src/core/rational.cc
index 1e440765f2..2581f35f68 100644
--- a/src/core/rational.cc
+++ b/src/core/rational.cc
@@ -562,6 +562,9 @@ template <> Rational lexical_cast(const std::string &f)
         expsign = -1;
         ch = f[index++];
       }
+      else if (ch == '+') {
+        ch = f[index++];
+      }
       while (ch >= '0' && ch <= '9' && index <= length) {
         exponent *= 10;
         exponent += (ch - '0');
@@ -589,6 +592,9 @@ template <> Rational lexical_cast(const std::string &f)
       expsign = -1;
       ch = f[index++];
     }
+    else if (ch == '+') {
+      ch = f[index++];
+    }
     while (ch >= '0' && ch <= '9' && index <= length) {
       exponent *= 10;
       exponent += (ch - '0');