diff --git a/Doc/Zsh/options.yo b/Doc/Zsh/options.yo
index 25b3d57..bc182eb 100644
--- a/Doc/Zsh/options.yo
+++ b/Doc/Zsh/options.yo
@@ -753,7 +753,9 @@ pindex(NOUNSET)
 cindex(parameters, substituting unset)
 cindex(unset parameters, substituting)
 item(tt(UNSET) (tt(PLUS()u), ksh: tt(PLUS()u)) <K> <S> <Z>)(
-Treat unset parameters as if they were empty when substituting.
+Treat unset parameters as if they were empty when substituting, and as if
+they were zero when reading their values in arithmetic expansion and
+arithmetic commands.
 Otherwise they are treated as an error.
 )
 pindex(WARN_CREATE_GLOBAL)
diff --git a/README b/README
index aaaee50..0002104 100644
--- a/README
+++ b/README
@@ -42,6 +42,10 @@ array.
 The gen-applied-string hook is unaffected; it still receives the patches in
 reverse order, from last applied to first applied.
 
+2) The option NO_UNSET now also applies when reading values from
+variables without a preceding '$' sign in shell arithmetic expansion
+and in the double-parentheses and 'let' arithmetic commands.
+
 Incompatibilities between 5.5.1 and 5.6.2
 ------------------------------------------
 
diff --git a/Src/math.c b/Src/math.c
index b08e05c..a387700 100644
--- a/Src/math.c
+++ b/Src/math.c
@@ -342,6 +342,8 @@ getmathparam(struct mathvalue *mptr)
 	mptr->pval = (Value)zhalloc(sizeof(struct value));
 	if (!getvalue(mptr->pval, &s, 1))
 	{
+	    if (unset(UNSET))
+		zerr("%s: parameter not set", mptr->lval);
 	    mptr->pval = NULL;
 	    if (isset(FORCEFLOAT)) {
 		result.type = MN_FLOAT;
diff --git a/Test/C01arith.ztst b/Test/C01arith.ztst
index f1364ab..9dfc065 100644
--- a/Test/C01arith.ztst
+++ b/Test/C01arith.ztst
@@ -471,3 +471,19 @@
   print $(( -2#101-16#f ))
 0: Unary minus doesn't apply to base but to number as a whole.
 >-20
+
+  ( set -o nounset
+  true $(( noexist + 1 ))
+  echo 'should never get here' )
+1:Arithmetic, NO_UNSET part 1
+?(eval):2: noexist: parameter not set
+
+  ( setopt nounset
+  (( noexist++ )) )
+2:Arithmetic, NO_UNSET part 2
+?(eval):2: noexist: parameter not set
+
+  ( unsetopt unset
+  let noexist==0 )
+1:Arithmetic, NO_UNSET part 3
+?(eval):2: noexist: parameter not set