🚨 Error: Cannot read property 'split' of undefined

Details
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**Error report:** [link](http://go/ampe/CKLf6JngstqzPQ)
**First seen:** Jul 10, 2020
**Frequency:** ~ 4,524/day

Stacktrace
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Error: Cannot read property 'split' of undefined

    at expr (src/json.js:85:16)

    at getValueForExpr (extensions/amp-bind/0.1/bind-impl.js:594:18)

    at bind (src/service/url-replacements-impl.js:597:15)

Notes
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`@dvoytenko` modified `src/json.js:85-88` in #1951 (Feb 12, 2016)
`@mdmower` modified `src/service/url-replacements-impl.js:597-597` in #26857 (Feb 19, 2020)

**Seen in:**
- 09-11 Control (2034+2)
- 09-11 Experimental (2034+2)
- 09-11 Stable (2034+2)
- 09-19 Experiment-A (0410)
- 09-19 Experimental (0410)
- +9 more

**Possible assignees:** `@mdmower`, `@dvoytenko`

/cc @ampproject/release-on-duty