diff --git a/vhdlpp/expression_elaborate.cc b/vhdlpp/expression_elaborate.cc
index 348b47cea..76850aafd 100644
--- a/vhdlpp/expression_elaborate.cc
+++ b/vhdlpp/expression_elaborate.cc
@@ -795,7 +795,9 @@ const VType*ExpFunc::probe_type(Entity*, ScopeBase*scope) const
           prog = library_find_subprogram(name_);
 
       if(!prog) {
-          cerr << get_fileline() << ": sorry: VHDL function " << name_ << " not yet implemented" << endl;
+          cerr << get_fileline() << ": sorry: could not find function ";
+          emit_subprogram_sig(cerr, name_, arg_types);
+          cerr << endl;
           ivl_assert(*this, false);
       }
 
@@ -812,8 +814,13 @@ int ExpFunc::elaborate_expr(Entity*ent, ScopeBase*scope, const VType*)
       if(!prog)
             prog = library_find_subprogram(name_);
 
-      ivl_assert(*this, def_==0);
       def_ = prog;
+      if(!def_) {
+            cerr << get_fileline() << ": error: could not find function ";
+            emit_subprogram_sig(cerr, name_, arg_types);
+            cerr << endl;
+            return 1;
+      }
 
 	// Elaborate arguments
       for (size_t idx = 0; idx < argv_.size(); ++idx) {
@@ -828,11 +835,6 @@ int ExpFunc::elaborate_expr(Entity*ent, ScopeBase*scope, const VType*)
             name_ = def_->name();
       }
 
-      if(!def_) {
-            cerr << get_fileline() << ": error: could not find function " << name_ << endl;
-            ++errors;
-      }
-
       return errors;
 }
 
diff --git a/vhdlpp/sequential_elaborate.cc b/vhdlpp/sequential_elaborate.cc
index d3e1e493a..4d67daa65 100644
--- a/vhdlpp/sequential_elaborate.cc
+++ b/vhdlpp/sequential_elaborate.cc
@@ -183,7 +183,12 @@ int ProcedureCall::elaborate(Entity*ent, ScopeBase*scope)
       if(!def_)
             def_ = library_find_subprogram(name_);
 
-      assert(def_);
+      if(!def_) {
+            cerr << get_fileline() << ": error: could not find procedure ";
+            emit_subprogram_sig(cerr, name_, arg_types);
+            cerr << endl;
+            return 1;
+      }
 
 	// Elaborate arguments
       size_t idx = 0;
diff --git a/vhdlpp/subprogram.h b/vhdlpp/subprogram.h
index ba10f6e56..6eb0446c9 100644
--- a/vhdlpp/subprogram.h
+++ b/vhdlpp/subprogram.h
@@ -169,4 +169,8 @@ class SubprogramBuiltin : public SubprogramStdHeader
       perm_string sv_name_;
 };
 
+// Helper function to print out a human-readable function signature.
+void emit_subprogram_sig(std::ostream&out, perm_string name,
+        const std::list<const VType*>&arg_types);
+
 #endif /* IVL_subprogram_H */
diff --git a/vhdlpp/subprogram_emit.cc b/vhdlpp/subprogram_emit.cc
index 18ab855a3..0aa02973e 100644
--- a/vhdlpp/subprogram_emit.cc
+++ b/vhdlpp/subprogram_emit.cc
@@ -142,3 +142,23 @@ int SubprogramBuiltin::emit_name(const std::vector<Expression*>&,
     out << sv_name_;
     return 0;
 }
+
+void emit_subprogram_sig(ostream&out, perm_string name,
+        const list<const VType*>&arg_types)
+{
+    out << name << "(";
+    bool first = true;
+    for(list<const VType*>::const_iterator it = arg_types.begin();
+            it != arg_types.end(); ++it) {
+        if(first)
+            first = false;
+        else
+            out << ", ";
+
+        if(*it)
+            (*it)->write_to_stream(out);
+        else
+            out << "<unresolved type>";
+    }
+    out << ")";
+}