diff --git a/js/src/jit-test/tests/parallel/bug1022948.js b/js/src/jit-test/tests/parallel/bug1022948.js
new file mode 100644
index 000000000000..26609610356d
--- /dev/null
+++ b/js/src/jit-test/tests/parallel/bug1022948.js
@@ -0,0 +1,12 @@
+if (!getBuildConfiguration().parallelJS)
+  quit(0);
+
+var a = Array.buildPar(999, function() {
+    return (Math.asin(Math.tanh(1)))
+});
+var count = 0;
+for (let e of a) {
+  count++;
+  print(count);
+  assertEq(e, a[0])
+}
diff --git a/js/src/jsnum.cpp b/js/src/jsnum.cpp
index e64a5300b161..5425134081d0 100644
--- a/js/src/jsnum.cpp
+++ b/js/src/jsnum.cpp
@@ -1105,26 +1105,22 @@ static JSConstDoubleSpec number_constants[] = {
     {0,0,0,{0,0,0}}
 };
 
-#if (defined __GNUC__ && defined __i386__) || \
-    (defined __SUNPRO_CC && defined __i386)
-
 /*
  * Set the exception mask to mask all exceptions and set the FPU precision
  * to 53 bit mantissa (64 bit doubles).
  */
-static inline void FIX_FPU() {
+void
+js::FIX_FPU()
+{
+#if (defined __GNUC__ && defined __i386__) || \
+    (defined __SUNPRO_CC && defined __i386)
     short control;
     asm("fstcw %0" : "=m" (control) : );
     control &= ~0x300; // Lower bits 8 and 9 (precision control).
     control |= 0x2f3;  // Raise bits 0-5 (exception masks) and 9 (64-bit precision).
     asm("fldcw %0" : : "m" (control) );
-}
-
-#else
-
-#define FIX_FPU() ((void)0)
-
 #endif
+}
 
 bool
 js::InitRuntimeNumberState(JSRuntime *rt)
diff --git a/js/src/jsnum.h b/js/src/jsnum.h
index f76ede5c7654..729b4563135c 100644
--- a/js/src/jsnum.h
+++ b/js/src/jsnum.h
@@ -332,6 +332,8 @@ NonObjectToUint32(ThreadSafeContext *cx, const Value &v, uint32_t *out)
     return NonObjectToUint32Slow(cx, v, out);
 }
 
+void FIX_FPU();
+
 } /* namespace js */
 
 #endif /* jsnum_h */
diff --git a/js/src/vm/ThreadPool.cpp b/js/src/vm/ThreadPool.cpp
index 3108685d375a..849fbcd721f0 100644
--- a/js/src/vm/ThreadPool.cpp
+++ b/js/src/vm/ThreadPool.cpp
@@ -9,6 +9,7 @@
 #include "mozilla/Atomics.h"
 
 #include "jslock.h"
+#include "jsnum.h" // for FIX_FPU
 
 #include "js/Utility.h"
 #include "vm/ForkJoin.h"
@@ -174,6 +175,10 @@ ThreadPoolWorker::HelperThreadMain(void *arg)
     }
 #endif
 
+    // Set the FPU control word to be the same as the main thread's, else we
+    // might get inconsistent results from math functions.
+    FIX_FPU();
+
     worker->helperLoop();
 }