diff --git a/src/rampl.cpp b/src/rampl.cpp
index 7a6ee3d..82bf0fc 100644
--- a/src/rampl.cpp
+++ b/src/rampl.cpp
@@ -326,7 +326,7 @@ void RAMPL::solve(Rcpp::Nullable<std::string> problem = R_NilValue, Rcpp::Nullab
   else if (solver.isNotNull())
     _impl.solve("", solver);
   else
-    _impl.solve("", "")
+    _impl.solve("", "");
   
   
   //return _impl.solve("", ""); // FIXME: does not print to stdout with R IDE on Windows
diff --git a/src/rampl.h b/src/rampl.h
index cb69d74..c9ce382 100644
--- a/src/rampl.h
+++ b/src/rampl.h
@@ -104,7 +104,7 @@ class RAMPL {
   void reset();
   void close();
   bool isRunning() const;
-  void solve(std::string problem="", std::string solver="");
+  void solve(Rcpp::Nullable<std::string> problem = R_NilValue, Rcpp::Nullable<std::string> solver = R_NilValue);
 
   Rcpp::DataFrame getData(Rcpp::List statements) const;
   SEXP getValue(std::string scalarExpression) const;