diff --git a/js/src/jit/arm/Assembler-arm.cpp b/js/src/jit/arm/Assembler-arm.cpp
index 74c0d59bc05..da15f9552cc 100644
--- a/js/src/jit/arm/Assembler-arm.cpp
+++ b/js/src/jit/arm/Assembler-arm.cpp
@@ -1427,7 +1427,7 @@ Assembler::bytesNeeded() const
 void
 Assembler::spew(Instruction* i)
 {
-    if (spewDisabled())
+    if (spewDisabled() || !i)
         return;
     disasm::NameConverter converter;
     disasm::Disassembler dasm(converter);
@@ -1453,7 +1453,7 @@ Assembler::spewTarget(Label* target)
 void
 Assembler::spewBranch(Instruction* i, Label* target /* may be nullptr */)
 {
-    if (spewDisabled())
+    if (spewDisabled() || !i)
         return;
     disasm::NameConverter converter;
     disasm::Disassembler dasm(converter);
@@ -1515,7 +1515,10 @@ Assembler::spewData(BufferOffset addr, size_t numInstr, bool loadToPC)
 {
     if (spewDisabled())
         return;
-    uint32_t *instr = reinterpret_cast<uint32_t*>(m_buffer.getInst(addr));
+    Instruction* inst = m_buffer.getInstOrNull(addr);
+    if (!inst)
+        return;
+    uint32_t *instr = reinterpret_cast<uint32_t*>(inst);
     for ( size_t k=0 ; k < numInstr ; k++ ) {
         spew("   %08x  %08x       (patchable constant load%s)",
              reinterpret_cast<uint32_t>(instr+k), *(instr+k), loadToPC ? " to PC" : "");
@@ -1636,7 +1639,7 @@ Assembler::writeInst(uint32_t x)
 {
     BufferOffset offs = m_buffer.putInt(x);
 #ifdef JS_DISASM_ARM
-    spew(m_buffer.getInst(offs));
+    spew(m_buffer.getInstOrNull(offs));
 #endif
     return offs;
 }
@@ -1646,7 +1649,7 @@ Assembler::writeBranchInst(uint32_t x, Label* documentation)
 {
     BufferOffset offs = m_buffer.putInt(x, /* markAsBranch = */ true);
 #ifdef JS_DISASM_ARM
-    spewBranch(m_buffer.getInst(offs), documentation);
+    spewBranch(m_buffer.getInstOrNull(offs), documentation);
 #endif
     return offs;
 }
@@ -2375,7 +2378,7 @@ Assembler::as_b(Label* l, Condition c)
         BufferOffset ret = allocBranchInst();
         as_b(BufferOffset(l).diffB<BOffImm>(ret), c, ret);
 #ifdef JS_DISASM_ARM
-        spewBranch(m_buffer.getInst(ret), l);
+        spewBranch(m_buffer.getInstOrNull(ret), l);
 #endif
         return ret;
     }
@@ -2442,7 +2445,7 @@ Assembler::as_bl(Label* l, Condition c)
         BufferOffset ret = allocBranchInst();
         as_bl(BufferOffset(l).diffB<BOffImm>(ret), c, ret);
 #ifdef JS_DISASM_ARM
-        spewBranch(m_buffer.getInst(ret), l);
+        spewBranch(m_buffer.getInstOrNull(ret), l);
 #endif
         return ret;
     }
diff --git a/js/src/jit/shared/IonAssemblerBuffer.h b/js/src/jit/shared/IonAssemblerBuffer.h
index bb852ffe656..be1251e45e2 100644
--- a/js/src/jit/shared/IonAssemblerBuffer.h
+++ b/js/src/jit/shared/IonAssemblerBuffer.h
@@ -279,6 +279,12 @@ class AssemblerBuffer
     }
 
   public:
+    Inst* getInstOrNull(BufferOffset off) {
+        if (!off.assigned())
+            return nullptr;
+        return getInst(off);
+    }
+
     Inst* getInst(BufferOffset off) {
         const int offset = off.getOffset();