diff --git a/lld/wasm/SyntheticSections.h b/lld/wasm/SyntheticSections.h
--- a/lld/wasm/SyntheticSections.h
+++ b/lld/wasm/SyntheticSections.h
@@ -101,14 +101,28 @@
   void writeBody() override;
   void addImport(Symbol *Sym);
   void addGOTEntry(Symbol *Sym);
+  void seal() { IsSealed = true; }
   uint32_t numImports() const;
+  uint32_t numImportedGlobals() const {
+    assert(IsSealed);
+    return NumImportedGlobals;
+  }
+  uint32_t numImportedFunctions() const {
+    assert(IsSealed);
+    return NumImportedFunctions;
+  }
+  uint32_t numImportedEvents() const {
+    assert(IsSealed);
+    return NumImportedEvents;
+  }
 
-  unsigned NumImportedGlobals = 0;
-  unsigned NumImportedFunctions = 0;
-  unsigned NumImportedEvents = 0;
   std::vector<const Symbol *> ImportedSymbols;
 
 protected:
+  bool IsSealed = false;
+  unsigned NumImportedGlobals = 0;
+  unsigned NumImportedFunctions = 0;
+  unsigned NumImportedEvents = 0;
   std::vector<const Symbol *> GOTSymbols;
 };
 
diff --git a/lld/wasm/SyntheticSections.cpp b/lld/wasm/SyntheticSections.cpp
--- a/lld/wasm/SyntheticSections.cpp
+++ b/lld/wasm/SyntheticSections.cpp
@@ -92,6 +92,7 @@
 }
 
 uint32_t ImportSection::numImports() const {
+  assert(IsSealed);
   uint32_t NumImports = ImportedSymbols.size() + GOTSymbols.size();
   if (Config->ImportMemory)
     ++NumImports;
@@ -101,6 +102,7 @@
 }
 
 void ImportSection::addGOTEntry(Symbol *Sym) {
+  //assert(!IsSealed);
   if (Sym->hasGOTIndex())
     return;
   Sym->setGOTIndex(NumImportedGlobals++);
@@ -108,6 +110,7 @@
 }
 
 void ImportSection::addImport(Symbol *Sym) {
+  assert(!IsSealed);
   ImportedSymbols.emplace_back(Sym);
   if (auto *F = dyn_cast<FunctionSymbol>(Sym))
     F->setFunctionIndex(NumImportedFunctions++);
@@ -202,7 +205,7 @@
   if (!Func->Live)
     return;
   uint32_t FunctionIndex =
-      Out.ImportSec->NumImportedFunctions + InputFunctions.size();
+      Out.ImportSec->numImportedFunctions() + InputFunctions.size();
   InputFunctions.emplace_back(Func);
   Func->setFunctionIndex(FunctionIndex);
 }
@@ -251,7 +254,7 @@
   if (!Global->Live)
     return;
   uint32_t GlobalIndex =
-      Out.ImportSec->NumImportedGlobals + InputGlobals.size();
+      Out.ImportSec->numImportedGlobals() + InputGlobals.size();
   LLVM_DEBUG(dbgs() << "addGlobal: " << GlobalIndex << "\n");
   Global->setGlobalIndex(GlobalIndex);
   Out.GlobalSec->InputGlobals.push_back(Global);
@@ -270,7 +273,7 @@
 void EventSection::addEvent(InputEvent *Event) {
   if (!Event->Live)
     return;
-  uint32_t EventIndex = Out.ImportSec->NumImportedEvents + InputEvents.size();
+  uint32_t EventIndex = Out.ImportSec->numImportedEvents() + InputEvents.size();
   LLVM_DEBUG(dbgs() << "addEvent: " << EventIndex << "\n");
   Event->setEventIndex(EventIndex);
   InputEvents.push_back(Event);
@@ -457,7 +460,7 @@
 }
 
 unsigned NameSection::numNames() const {
-  unsigned NumNames = Out.ImportSec->NumImportedFunctions;
+  unsigned NumNames = Out.ImportSec->numImportedFunctions();
   for (const InputFunction *F : Out.FunctionSec->InputFunctions)
     if (!F->getName().empty() || !F->getDebugName().empty())
       ++NumNames;
diff --git a/lld/wasm/Writer.cpp b/lld/wasm/Writer.cpp
--- a/lld/wasm/Writer.cpp
+++ b/lld/wasm/Writer.cpp
@@ -439,7 +439,7 @@
         WasmExport{FunctionTableName, WASM_EXTERNAL_TABLE, 0});
 
   unsigned FakeGlobalIndex =
-      Out.ImportSec->NumImportedGlobals + Out.GlobalSec->InputGlobals.size();
+      Out.ImportSec->numImportedGlobals() + Out.GlobalSec->InputGlobals.size();
 
   for (Symbol *Sym : Symtab->getSymbols()) {
     if (!Sym->isExported())
@@ -532,7 +532,9 @@
 }
 
 void Writer::assignIndexes() {
-  assert(Out.FunctionSec->InputFunctions.empty());
+  // Seal the import section, since other index spaces such as function and
+  // global are effected by the number of imports.
+  Out.ImportSec->seal();
 
   for (InputFunction *Func : Symtab->SyntheticFunctions)
     Out.FunctionSec->addFunction(Func);
@@ -750,9 +752,9 @@
     log("Defined Functions: " + Twine(Out.FunctionSec->InputFunctions.size()));
     log("Defined Globals  : " + Twine(Out.GlobalSec->InputGlobals.size()));
     log("Defined Events   : " + Twine(Out.EventSec->InputEvents.size()));
-    log("Function Imports : " + Twine(Out.ImportSec->NumImportedFunctions));
-    log("Global Imports   : " + Twine(Out.ImportSec->NumImportedGlobals));
-    log("Event Imports    : " + Twine(Out.ImportSec->NumImportedEvents));
+    log("Function Imports : " + Twine(Out.ImportSec->numImportedFunctions()));
+    log("Global Imports   : " + Twine(Out.ImportSec->numImportedGlobals()));
+    log("Event Imports    : " + Twine(Out.ImportSec->numImportedEvents()));
     for (ObjFile *File : Symtab->ObjectFiles)
       File->dumpInfo();
   }