diff --git a/install.sh b/install.sh
index 6dc692b..36b0916 100755
--- a/install.sh
+++ b/install.sh
@@ -121,7 +121,24 @@ __am_prompt_install() {
 
     echo $PROMPT_SHA > $PROMPT_SHA_PATH
 
-    local PROMPT_SHELL="${1:-"bash"}"
+    # get the requested shell
+    local PROMPT_SHELL="${1:-}"
+
+    # determine if the request shell was not set
+    if [ -z "${PROMPT_SHELL:-}" ]; then
+
+        # get the current shell
+        PROMPT_SHELL=$(echo $SHELL | rev | cut -d'/' -f1 | rev)
+
+        # test if the current shell is something other than zsh
+        if [ "$PROMPT_SHELL" != "zsh" ]; then
+
+            # default to bash
+            PROMPT_SHELL="bash"
+        fi
+    fi
+
+    # lowercase the prompt shell
     local PROMPT_SHELL=$(echo $PROMPT_SHELL | tr '[:upper:]' '[:lower:]')
 
     # use the correct shell
diff --git a/src/bash/bashrc b/src/bash/bashrc
index 2bf30eb..4b5e1a2 100755
--- a/src/bash/bashrc
+++ b/src/bash/bashrc
@@ -1,5 +1,8 @@
 #!/usr/bin/env bash
 
+# set prompt shell
+export PROMPT_SHELL="bash"
+
 # shell options
 shopt -s checkwinsize     # check window size on script exit
 shopt -s cdable_vars      # enable change directory to a variable value
diff --git a/src/zsh/zshrc b/src/zsh/zshrc
index 547a665..cf431a8 100644
--- a/src/zsh/zshrc
+++ b/src/zsh/zshrc
@@ -1,5 +1,8 @@
 #!/usr/bin/env zsh
 
+# set prompt shell
+export PROMPT_SHELL="zsh"
+
 # determine if this is a tty
 if [ -t 1 ]; then