--- a/js/src/vm/CodeCoverage.cpp
+++ b/js/src/vm/CodeCoverage.cpp
@@ -69,23 +69,29 @@ LCovSource::LCovSource(LifoAlloc* alloc,
     outFNDA_(alloc),
     numFunctionsFound_(0),
     numFunctionsHit_(0),
     outBRDA_(alloc),
     numBranchesFound_(0),
     numBranchesHit_(0),
     outDA_(alloc),
     numLinesInstrumented_(0),
-    numLinesHit_(0)
+    numLinesHit_(0),
+    hasFilename_(false),
+    hasScripts_(false)
 {
 }
 
 void
 LCovSource::exportInto(GenericPrinter& out) const
 {
+    // Only write if everything got recorded.
+    if (!hasFilename_ || !hasScripts_)
+        return;
+
     outSF_.exportInto(out);
 
     outFN_.exportInto(out);
     outFNDA_.exportInto(out);
     out.printf("FNF:%d\n", numFunctionsFound_);
     out.printf("FNH:%d\n", numFunctionsHit_);
 
     outBRDA_.exportInto(out);
@@ -151,27 +157,37 @@ LCovSource::writeTopLevelScript(JSScript
 
             // Queue the script in the list of script associated to the
             // current source.
             if (!queue.append(fun.nonLazyScript()))
                 return false;
         }
     } while (!queue.empty());
 
-    return !(outFN_.hadOutOfMemory() ||
-             outFNDA_.hadOutOfMemory() ||
-             outBRDA_.hadOutOfMemory() ||
-             outDA_.hadOutOfMemory());
+    if (outFN_.hadOutOfMemory() ||
+        outFNDA_.hadOutOfMemory() ||
+        outBRDA_.hadOutOfMemory() ||
+        outDA_.hadOutOfMemory())
+    {
+        return false;
+    }
+
+    hasScripts_ = true;
+    return true;
 }
 
 bool
 LCovSource::writeSourceFilename(ScriptSourceObject* sso)
 {
     outSF_.printf("SF:%s\n", sso->source()->filename());
-    return !outSF_.hadOutOfMemory();
+    if (outSF_.hadOutOfMemory())
+        return false;
+
+    hasFilename_ = true;
+    return true;
 }
 
 bool
 LCovSource::writeScriptName(LSprinter& out, JSScript* script)
 {
     JSFunction* fun = script->functionNonDelazifying();
     if (fun && fun->displayAtom())
         return EscapedStringPrinter(out, fun->displayAtom(), 0);
@@ -306,22 +322,22 @@ LCovCompartment::collectCodeCoverageInfo
     // Skip any operation if we already some out-of memory issues.
     if (outTN_.hadOutOfMemory())
         return;
 
     // We expect to visit the source before visiting any of its scripts.
     if (!sources_)
         return;
 
-    // Lookup if there is already one entry.
-    LCovSource* source = lookup(sso);
+    // Get the existing source LCov summary, or create a new one.
+    LCovSource* source = lookupOrAdd(comp, sso);
     if (!source)
         return;
 
-    // Write code coverage data into the allocated LCovSource.
+    // Write code coverage data into the LCovSource.
     if (!source->writeTopLevelScript(topLevel)) {
         outTN_.reportOutOfMemory();
         return;
     }
 }
 
 void
 LCovCompartment::collectSourceFile(JSCompartment* comp, ScriptSourceObject* sso)
@@ -329,57 +345,59 @@ LCovCompartment::collectSourceFile(JSCom
     // Do not add sources if there is no file name associated to it.
     if (!sso->source()->filename())
         return;
 
     // Skip any operation if we already some out-of memory issues.
     if (outTN_.hadOutOfMemory())
         return;
 
-    // On the first call, write the compartment name, and allocate a LCovSource
-    // vector in the LifoAlloc.
-    if (!sources_) {
-        if (!writeCompartmentName(comp))
-            return;
-
-        LCovSourceVector* raw = alloc_.pod_malloc<LCovSourceVector>();
-        if (!raw) {
-            outTN_.reportOutOfMemory();
-            return;
-        }
+    // Get the existing source LCov summary, or create a new one.
+    LCovSource* source = lookupOrAdd(comp, sso);
+    if (!source)
+        return;
 
-        sources_ = new(raw) LCovSourceVector(alloc_);
-    }
-
-    // Allocate a new LCovSource for the current top-level.
-    if (!sources_->append(Move(LCovSource(&alloc_, sso)))) {
-        outTN_.reportOutOfMemory();
-        return;
-    }
-
-    // Write code coverage data into the allocated LCovSource.
-    if (!sources_->back().writeSourceFilename(sso)) {
+    // Write source filename into the LCovSource.
+    if (!source->writeSourceFilename(sso)) {
         outTN_.reportOutOfMemory();
         return;
     }
 }
 
 LCovSource*
-LCovCompartment::lookup(JSObject* sso)
+LCovCompartment::lookupOrAdd(JSCompartment* comp, JSObject* sso)
 {
-    if (!sources_)
-        return nullptr;
+    // On the first call, write the compartment name, and allocate a LCovSource
+    // vector in the LifoAlloc.
+    if (!sources_) {
+        if (!writeCompartmentName(comp))
+            return nullptr;
 
-    // Find the first matching source.
-    for (LCovSource& source : *sources_) {
-        if (source.match(sso))
-            return &source;
+        LCovSourceVector* raw = alloc_.pod_malloc<LCovSourceVector>();
+        if (!raw) {
+            outTN_.reportOutOfMemory();
+            return nullptr;
+        }
+
+        sources_ = new(raw) LCovSourceVector(alloc_);
+    } else {
+        // Find the first matching source.
+        for (LCovSource& source : *sources_) {
+            if (source.match(sso))
+                return &source;
+        }
     }
 
-    return nullptr;
+    // Allocate a new LCovSource for the current top-level.
+    if (!sources_->append(Move(LCovSource(&alloc_, sso)))) {
+        outTN_.reportOutOfMemory();
+        return nullptr;
+    }
+
+    return &sources_->back();
 }
 
 void
 LCovCompartment::exportInto(GenericPrinter& out) const
 {
     if (!sources_ || outTN_.hadOutOfMemory())
         return;
 

--- a/js/src/vm/CodeCoverage.h
+++ b/js/src/vm/CodeCoverage.h
@@ -76,16 +76,20 @@ class LCovSource
     LSprinter outBRDA_;
     size_t numBranchesFound_;
     size_t numBranchesHit_;
 
     // LifoAlloc string which hold lines statistics.
     LSprinter outDA_;
     size_t numLinesInstrumented_;
     size_t numLinesHit_;
+
+    // Status flags.
+    bool hasFilename_ : 1;
+    bool hasScripts_ : 1;
 };
 
 class LCovCompartment
 {
   public:
     LCovCompartment();
 
     // Collect code coverage information for the given source.
@@ -98,17 +102,17 @@ class LCovCompartment
     // the runtime code coverage trace file.
     void exportInto(GenericPrinter& out) const;
 
   private:
     // Write the script name in out.
     bool writeCompartmentName(JSCompartment* comp);
 
     // Return the LCovSource entry which matches the given ScriptSourceObject.
-    LCovSource* lookup(JSObject* sso);
+    LCovSource* lookupOrAdd(JSCompartment* comp, JSObject* sso);
 
   private:
     typedef Vector<LCovSource, 16, LifoAllocPolicy<Fallible>> LCovSourceVector;
 
     // LifoAlloc backend for all temporary allocations needed to stash the
     // strings to be written in the file.
     LifoAlloc alloc_;