diff --git a/base/docs/helpdb/Base.jl b/base/docs/helpdb/Base.jl
index 43c995fac199ff..134e929dbf54cd 100644
--- a/base/docs/helpdb/Base.jl
+++ b/base/docs/helpdb/Base.jl
@@ -1675,12 +1675,17 @@ eta
 """
     isdefined([m::Module,] s::Symbol)
     isdefined(object, s::Symbol)
-    isdefined(a::AbstractArray, index::Int)
+    isdefined(object, index::Int)
+    isdefined(a::Array, index::Int)
 
 Tests whether an assignable location is defined. The arguments can be a module and a symbol,
-a composite object and field name (as a symbol), or an array and index. With a single
-symbol argument, tests whether a global variable with that name is defined in
+a composite object and field name (as a symbol) or index, or an Array and index.
+With a single symbol argument, tests whether a global variable with that name is defined in
 `current_module()`.
+
+Note: For AbstractArrays other than Array, `isdefined` tests whether the given field index
+is defined, not the given array index. To test whether an array index is defined, use
+[`isassigned`](:func:`isassigned`).
 """
 isdefined
 
diff --git a/doc/stdlib/base.rst b/doc/stdlib/base.rst
index 76f85d5b93af66..f0c93a4dd91a20 100644
--- a/doc/stdlib/base.rst
+++ b/doc/stdlib/base.rst
@@ -349,11 +349,14 @@ All Objects
 
 .. function:: isdefined([m::Module,] s::Symbol)
               isdefined(object, s::Symbol)
-              isdefined(a::AbstractArray, index::Int)
+              isdefined(object, index::Int)
+              isdefined(a::Array, index::Int)
 
    .. Docstring generated from Julia source
 
-   Tests whether an assignable location is defined. The arguments can be a module and a symbol, a composite object and field name (as a symbol), or an array and index. With a single symbol argument, tests whether a global variable with that name is defined in ``current_module()``\ .
+   Tests whether an assignable location is defined. The arguments can be a module and a symbol, a composite object and field name (as a symbol) or index, or an Array and index. With a single symbol argument, tests whether a global variable with that name is defined in ``current_module()``\ .
+
+   Note: For AbstractArrays other than Array, ``isdefined`` tests whether the given field index is defined, not the given array index. To test whether an array index is defined, use :func:`isassigned`\ .
 
 .. function:: convert(T, x)