gap_ppcp_set invalid parameters

[q]

My call to sd_ble_gap_ppcp_set is returning NRF_ERROR_INVALID_PARAMETER.

I have the following connection interval settings:

#define MIN_CONN_INTERVAL                MSEC_TO_UNITS(500, UNIT_1_25_MS)
#define MAX_CONN_INTERVAL                MSEC_TO_UNITS(1000, UNIT_1_25_MS)
#define SLAVE_LATENCY                    3
#define CONN_SUP_TIMEOUT                 MSEC_TO_UNITS(2000, UNIT_10_MS)

When initializing gap parameters in main.c, I run the following code:

gap_conn_params.min_conn_interval = MIN_CONN_INTERVAL;
gap_conn_params.max_conn_interval = MAX_CONN_INTERVAL;
gap_conn_params.slave_latency     = SLAVE_LATENCY;
gap_conn_params.conn_sup_timeout  = CONN_SUP_TIMEOUT;

err_code = sd_ble_gap_ppcp_set(&gap_conn_params);
APP_ERROR_CHECK(err_code);

Since it's a softdevice function, I can't see where in the function this error is being thrown. Any ideas/where I can find documentation that might help me?

If I use a shorter MIN_CONN_INTERVAL, the problem goes away, but 1) I want this longer connection interval 2) I've seen tons of example code with 500/1000 connection intervals.

PCA10040 RNF52832 SDK13