diff --git a/rts/Lua/LuaSyncedCtrl.cpp b/rts/Lua/LuaSyncedCtrl.cpp
index e2c9fcd54b5..c3f66d3a08f 100644
--- a/rts/Lua/LuaSyncedCtrl.cpp
+++ b/rts/Lua/LuaSyncedCtrl.cpp
@@ -1260,7 +1260,7 @@ int LuaSyncedCtrl::CreateUnit(lua_State* L)
 	if (unit == nullptr)
 		return 0;
 
-	unit->SetSoloBuilder(builder);
+	unit->SetSoloBuilder(builder, unitDef);
 
 	lua_pushnumber(L, unit->id);
 	return 1;
diff --git a/rts/Sim/Units/Unit.cpp b/rts/Sim/Units/Unit.cpp
index fff58bfa77a..1789f7c1714 100644
--- a/rts/Sim/Units/Unit.cpp
+++ b/rts/Sim/Units/Unit.cpp
@@ -1770,15 +1770,15 @@ void CUnit::DropCurrentAttackTarget()
 }
 
 
-void CUnit::SetSoloBuilder(CUnit* builder)
+bool CUnit::SetSoloBuilder(CUnit* builder, const UnitDef* buildeeDef)
 {
 	if (builder == nullptr)
-		return;
-	if (unitDef->canBeAssisted)
-		return;
+		return false;
+	if (buildeeDef->canBeAssisted)
+		return false;
 
-	soloBuilder = builder;
-	AddDeathDependence(builder, DEPENDENCE_BUILDER);
+	AddDeathDependence(soloBuilder = builder, DEPENDENCE_BUILDER);
+	return true;
 }
 
 void CUnit::SetLastAttacker(CUnit* attacker)
diff --git a/rts/Sim/Units/Unit.h b/rts/Sim/Units/Unit.h
index 98b3664cfc4..db0e67324de 100644
--- a/rts/Sim/Units/Unit.h
+++ b/rts/Sim/Units/Unit.h
@@ -198,7 +198,7 @@ class CUnit : public CSolidObject
 		int piece;
 	};
 
-	void SetSoloBuilder(CUnit* builder);
+	bool SetSoloBuilder(CUnit* builder, const UnitDef* buildeeDef);
 	void SetLastAttacker(CUnit* attacker);
 
 	void SetTransporter(CUnit* trans) { transporter = trans; }
diff --git a/rts/Sim/Units/UnitTypes/Builder.cpp b/rts/Sim/Units/UnitTypes/Builder.cpp
index 696c62fd24e..e45c0815e5e 100644
--- a/rts/Sim/Units/UnitTypes/Builder.cpp
+++ b/rts/Sim/Units/UnitTypes/Builder.cpp
@@ -120,7 +120,7 @@ bool CBuilder::CanAssistUnit(const CUnit* u, const UnitDef* def) const
 	if (!unitDef->canAssist)
 		return false;
 
-	return ((def == nullptr || u->unitDef == def) && u->beingBuilt && (u->buildProgress < 1.0f) && (!u->soloBuilder || u->soloBuilder == this));
+	return ((def == nullptr || u->unitDef == def) && u->beingBuilt && (u->buildProgress < 1.0f) && (u->soloBuilder == nullptr || u->soloBuilder == this));
 }
 
 
@@ -427,7 +427,7 @@ bool CBuilder::UpdateResurrect(const Command& fCommand)
 		CUnit* resurrectee = unitLoader->LoadUnit(resurrecteeParams);
 
 		assert(resurrecteeDef == resurrectee->unitDef);
-		resurrectee->SetSoloBuilder(this);
+		resurrectee->SetSoloBuilder(this, resurrecteeDef);
 
 		// TODO: make configurable if this should happen
 		resurrectee->health *= 0.05f;
@@ -780,7 +780,10 @@ bool CBuilder::StartBuild(BuildInfo& buildInfo, CFeature*& feature, bool& waitSt
 		terraformCenter = buildee->pos;
 	}
 
-	buildee->SetSoloBuilder(this);
+	// pass the *builder*'s udef for checking canBeAssisted; if buildee
+	// happens to be a non-assistable factory then it would also become
+	// impossible to *construct* with multiple builders
+	buildee->SetSoloBuilder(this, this->unitDef);
 	AddDeathDependence(curBuild = buildee, DEPENDENCE_BUILD);
 
 	// if the ground is not going to be terraformed the buildee would