diff --git a/src/libutil/error.cc b/src/libutil/error.cc
index 0765a2945..59628d875 100644
--- a/src/libutil/error.cc
+++ b/src/libutil/error.cc
@@ -3,6 +3,7 @@
 #include <iostream>
 #include <optional>
 #include "serialise.hh"
+#include <sstream>
 
 namespace nix {
 
@@ -16,6 +17,21 @@ BaseError & BaseError::addPrefix(const FormatOrString & fs)
     return *this;
 }
 
+const string& BaseError::calcWhat() const
+{
+    if (what_.has_value())
+        return *what_;
+    else {
+        err.name = sname();
+
+        std::ostringstream oss;
+        oss << err;
+        what_ = oss.str();
+
+        return *what_;
+    }
+}
+
 std::optional<string> ErrorInfo::programName = std::nullopt;
 
 std::ostream& operator<<(std::ostream &os, const hintformat &hf)
diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index 19c6e35a1..b374c2780 100644
--- a/src/libutil/error.hh
+++ b/src/libutil/error.hh
@@ -88,20 +88,8 @@ protected:
     mutable ErrorInfo err;
 
     mutable std::optional<string> what_;
-    const string& calcWhat() const
-    {
-        if (what_.has_value())
-            return *what_;
-        else {
-            err.name = sname();
-
-            std::ostringstream oss;
-            oss << err;
-            what_ = oss.str();
-
-            return *what_;
-        }
-    }
+    const string& calcWhat() const;
+    
 public:
     unsigned int status = 1; // exit status
 
diff --git a/src/libutil/types.hh b/src/libutil/types.hh
index 66072e8f7..89ae108f9 100644
--- a/src/libutil/types.hh
+++ b/src/libutil/types.hh
@@ -6,7 +6,7 @@
 #include <list>
 #include <set>
 #include <map>
-
+#include <vector>
 
 namespace nix {